Eli5: Wayland is a new way for Linux to make things appear on your screen, things like windows, graphics, and even your whole desktop.
The old way was called Xorg. It has been around for a very long time, and works pretty well, but the code is veryyyy messy and many developers wanted a new and easier way to make things appear on your screen, so they made Wayland.
(It's debatable how much easier Wayland actually is vs Xorg, but at this point most major distros are switching over to Wayland, so it's a moot point)