Not a stupid question at all. Here’s the Wikipedia article for it. The significant part is this:

The 5-dimensional discs [have] tiny patterns printed on 3 layers within the discs. Depending on the angle they are viewed from, these patterns can look completely different. This may sound like science fiction, but it’s basically a really fancy optical illusion. In this case, the 5 dimensions inside of the discs are the size and orientation in relation to the 3-dimensional position of the nanostructures. The concept of being 5-dimensional means that one disc has several different images depending on the angle that one views it from, and the magnification of the microscope used to view it. Basically, each disc has multiple layers of micro and macro level images.

Wavelength could add a dimension. For example, if you have an optical disc (2D) that can be read and written separately by red and blue lasers, that makes it 3D.