zram, formerly called compcache, is a Linux kernel module for creating a compressed block device in RAM, i.e. a RAM disk with on-the-fly disk compression. The block device created with zram can then be used for swap or as a general-purpose RAM disk. The two most common uses for zram are for the storage of temporary files (/tmp) and as a swap device.
It’s a Linux feature to compress some of your memory and then store it in a RAM disk. For systems with limited memory, it makes it go further.
https://en.wikipedia.org/wiki/Zram