Wayland is a windowing system, sort of. It's a replacement for X11. It changes how graphics are done on Linux machines, and has a more modern and secure approach which offers better performance and efficiency. It notoriously doesn't work very well on NVIDIA, because NVIDIA for a long time didn't allow the community to write drivers for Linux and didn't want to put effort into making it work correctly. This has changed recently but there are still some headaches with certain desktop environments or window managers.