The game does a really good job of backfilling information as you need it. Hover your mouse over any highlighted word and it’ll give you a short wiki entry. It’s a very approachable version of the setting.

The big stuff to grok right out of the gate is really just this:

• The Imperium of man is explicitly fascist. They’re not the good guys, but no one else is either. There are good people within this world, but there’s no “Hero” faction. Everything you see extolling the virtues of humanity in the setting is just imperial propaganda and hagiography, and you’ll pretty quickly start to see that play out within the game.
• You’ve pretty much nailed how Rogue Traders work. They basically act as the frontier of the Imperium, to the point of being allowed to colonise whole worlds, which they then own. Each Rogue Trader family is basically a noble house.
• The Adeptus Mechanicus understand the how, but not so much the why. So, they know how to repair a generator, but they believe that the process involves channeling the “motive force” through the wires. Most of what they do is carefully practiced methodology wrapped up in ritual. This isn’t true across the board however; at the higher levels of the mechanicus you do get people who actually know how to do real science. They’re just very rare. It’s mostly the guys who are like 10,000 years old.
• As mentioned by others, the big foundational thing is the Horus Heresy. Half of the space marine legions turned to the worship of the gods of Chaos, and tried to overthrow the emperor. It’s kind of both super important, and actually pretty irrelevant. Like, there are something like 40 fiction books detailing every moment of the heresy, but it’s impact on the setting now mostly just boils down to “This is why the emperor is a corpse on life support and why there are evil space marines.”
• Because of the warp, a realm where the line between imagined and real ceases to exist, there’s a lot of “Well I guess this might as well be magic” in the setting. Gods are real. Demons are real. People with the ability to wield magical powers are real; they might be called sorcerers and witches, or they might be called “psykers” depending on who you talk to, but it’s all the same stuff; pulling power from the warp to alter reality. This “magic” underpins a lot of the setting. People with warp abilities are necessary for long range communication and FTL travel.

If you’re familiar with Dune or Foundation you’ll notice that the setting borrows liberally from both properties, which give you some solid points of reference to draw from.

Other people have mostly covered it so I’ll just add that its foundational to the game to know that it is always funnier to have Abelard introduce you

I guess it’s probably helpful to know what a rogue trader actually is. I think the game explains it somewhat, but just in case, here’s a (not so) quick rundown on some major components of the lore and game.

A lot of people think that rogue traders are just the captain of a big ship. They are not. They command nearly unimaginable power within the imperium. They have a warrant of trade sealed with the blood of the emperor from a time when he walked among men. It is both a holy relic, and a hall pass to do whatever the fuck they want mostly. They are pretty much outside the bounds of imperial law. They can recruit xenos and worse into their crew. They can befriend, dominate, subjugate, and exterminate entire systems if they wish. Not even the Inquisition can really boss them around, but Lord inquisitors are probably the only authority to actually influence a rogue trader. A rogue trader wouldn’t want to piss off a lord inquisitor and vice versa.

It is the duty of rogue traders to navigate unexplored space and claim worlds for the imperium. The world’s they claim are theirs by right of their warrant of trade, but those worlds are required to pay tithe to the larger imperium, which is non negotiable. The tithe is usually set by the high lords of terra or the administrators that work underneath them.

As for wizards/magicians, they do and don’t exist. They are called psykers, and their “magic” is derived from the warp, the nightmarish parallel reality inhabited by daemons and their respective gods. Using warp energy is extremely dangerous, and can cause the veil between realities to thin. If it thins, tears open up, and daemons can burst forth. This is usually a one way street, as anything non-daemon entering, or even seeing the warp, will pretty much instantly go insane, turn into a chaos spawn, or worse. There are very few exceptions, but they do exist. There is a lot more to the warp, I’d recommend looking into it deeper if you’re interested. Leutin09 on YouTube is a great source.

Now that we know what both rogue traders are and what psykers are, we can talk about navigators, which are a really important part of the game and lore. They are human, but mutated. They have a genetic trait that gives them a third eye on their forehead. They use this eye to see into the warp. It’s not perfect, but it can help ships travel through the warp, which is humanity’s form of faster-than-light travel. Pretty much every warp capable craft will have one. Ones that don’t take extreme risks even doing the smallest warp jumps. Without navigators, the imperium is screwed. It’s why they are one of the very few types of sanctioned abhumans.

As for the mechanicus, machines spirits, and the god they worship, it’s somewhat complex. The mechanicus at its height was technologically progressive, and significantly more advanced than the contemporary version. They understood how to create and innovate. During the age of strife, they lost much of their knowledge. Creations they once built, they have lost the knowledge to rebuild, repair, enhance, and sometimes even maintain. When dark age technology is lost, it is usually lost for good, which makes it so valuable. The current day mechanicus has stagnated, and are afraid to progress as innovation has caused problems in the past (see Age of Strife and men of iron).

The modern day mechanicus warships the omnissiah, which is essentially the avatar of the machine god, which they believe is the emperor. In a roundabout way, they do workshop the emperor, which is relatively in line with the imperial creed. The mechanics is technically separate from the imperium per the treaty of Mars that the emperor signed, which gives the mechanicus religious freedoms and rights to technological finds, such a STC fragments. I’m not going to talk about STCs here. Mechanicus likes the fragments. They struck a deal with big E, and now they are an allied faction to the imperium. For all intents and purposes, you can consider them a joint faction, but they are technically distinct.

As for machine spirits, they do exist. I think of it kind of like how orc psychic manifestions work, though, to be honest I’m foggy on the details as machine spirits did exist during humanity’s enlightened phase. Technology does have machine spirits, and the more powerful the machine, the more powerful the machine spirit. Tech priests sometimes must commune with and appease machine spirits. Angry machine spirits can cause their host machine to misfire, break down, malfunction, etc. Appeased machine spirits can greatly enhance the effectiveness of the machine they are bound to, with some spirits being so powerful they can control aspects of a machine, such as advanced targetting systems, defensive capabilities, etc.

The last thing I’ll touch on is the emperor. Some comments say he’s dead, some say he’s alive. The truth is that he is neither. He is a husk that resides on the golden throne - a dark age of technology device that is a psychic amplifier and a life support system. It maintains his decript body and allows him to remain in a state of undeath. His psychic abilities are so powerful that he cannot really die. There is a lot more to this, as the emperor is a perpetual. I can expand on this more if you want, but I’ll leave it out for now as it’s complicated. Anyway, he is so powerful a psyker that he is the focal point or source of the astronomicon. This is essentially a psychic lighthouse in the warp that navigators will use a point of reference to know which way holy Terra is. They also say the emperor wages and eternal psychic war in the warp with the chaos gods, which keeps chaos from taking over the imperium of man.

You’ve got the gist of the foundation of the lore down, so my breakdown above is a little more comprehensive. Feel free to ask any more questions or DM me. I also wrote all this at 2am in the dark so if there are typos, inconsistencies, or things that need further clarification, I apologize and in advance.

I’m only on the periphery of the fandom, mostly just watch lore videos and play a few video games. The AdMech is my favorite faction. As for how real the machine cult is, it seems to depend heavily on the writer. My personal head canon is that the machine god is an actual warp entity like Isha and the other Eldar gods before Saanesh nommed them. If emotions and beliefs manifest as warp entities, then the Omnissiah should be one regardless of how the AdMech came to believe in it.

The Emperor of Mankind preached the Imperial Truth: Atheism, Logic, and Humanity. There are no gods. Which is a lie because gods do exist.

Then he was betrayed, mostly killed, and enshrined on the Golden Throne where 10,000 psychics are sacrificed per day to keep him alive.

And then his empire started preaching the Imperial Creed: That he is the God Emperor. A title he tried desperately to avoid.

Watch / listen to “If The Emperor had a Text To Speech.” It is a really good non canon primer for the world. It wasn’t made by GW, and so the creators abandoned that series a few years ago when GW changed their licence agreement.

It is about 90% canon, as in, if the Emperor could routinely talk to anyone, he would start unfucking The Imperium, post haste. The current Imperium is nothing like what he was trying to make.

The chaos factions are devoted to passion, life, knowledge and pleasure, and the Imperium are devoted to the corpse of a lying megalomaniac telling everyone they are forbidden from having a good time.

Anything else you hear is propaganda.

You’ll see a lot of brutal shit like servitors (lobotomized humans turned into mechanical servants). The origin for that is that AI was once very much in use, but the computers rose up and threatened humanity’s survival, so they made it very illegal. They now use “AI”, or “Abominable Intelligence”: living humans who have been turned into computers.

Another thing that’s interesting is that humanity’s been in the dark ages for so long that nobody has any idea how their shit works anymore. So they have people, called “tech priests” who dedicate their lives to learning voodoo bullshit to keep the machines running. Essentially, they believe that the machines are imbued with souls and they have to perform certain rituals to keep the machine spirits happy. All the giant military hardware they have is thousands of years old because nobody has any fucking clue how to manufacture it anymore. Same thing for most of their computer systems.

You’ve got the important foundational elements. The other big event is the Horus Heresy, where about half of the Emperor’s sons fell to Chaos and started a huge civil war that left the Emperor in his current half-dead state, but the imperial bureaucracy keeps on chugging.

The other thing to keep in mind is that the world of Warhammer 40k has been evolving over decades of novels, tabletop games, and video games made by multiple authors. As a result, the canon is vast and occasionally self-contradictory.

If you want to know more, I highly recommend the Ciaphas Cain books (novels with a comedy streak) and a silly fan series called If the Emperor Had a Text-to-Speech device.

I think reading the opening text on any book fully gives you a good grasp. Often it’s abbreviated to „in the grim darkness of the far future, there is only war“ All the other lines explain quite well, why humanity would be so inhumane in this scenario and why there can be only war. Basically all factions but especially humans live in different flavors of fascism in 40k. Orks might be a bit of an outlier here. Warhammer is a fun tabletop game and has awesome pc games and books. The lore is (or used to be) very critical and satirical on where humankind might end up in a science denying, fascist dystopia (basically the opposite of Star Trek)

Despite all criticism and satire, Warhammer itself is difficult on misogyny (e.g. only men can be space marines ™ because of genes), toxic masculinity & warrior culture, queer (with some positive exceptions) and trans topics (again, genes and also demons get mixed up here). Sadly the lore wise intergalactic xenophobia and racism also seems to be an open door to neo-nazis who turn the hobby into their racist fantasy. (Something I didn’t grasp but quickly realized looking at some armies of other players on tabletop and them talking).

Bricky on YouTube has a good series of videos talking about the lore of WH40K

https://youtu.be/zPB3Q59vjR8

https://youtu.be/xCGKPRiJp84

https://youtu.be/Y6jnsX77TCU

https://youtu.be/Ae2Beeleswc

Go down the rabbit hole on Youtube of Warhammer world lore and history. It’s pretty wild and fascinating.

The humans are not the good guys and the “God-Emperor” has actually died a long time ago.

More dakka.

That Ork stuff is a meme. Their tech works, it’s not powered by imagination.