It's a very long story, but yeah, its about how the applications on your desktop can draw content inside their borders, and how those applications interact with things like keyboard/mouse/other applications drawn on the screen. One of the big things is that X11 was written for a time where everything was open with no access control. Any application can view any other application, or capture any keystroke. Wayland introduces a form for permissions control, which has been a huge ordeal to both pave the path for and to design and figure out how on earth that should work. It's great though that you can have some actual control on who can listen to keystrokes and not.