Wayland is a "display server," which basically means it manages the way GUIs show on the screen. X (most recently X11/Xorg) was the standard for over 30 years, but it was designed for computers 30 years ago. Modern concepts like scaling and high refresh rate displays need extensions to it, but it's really complicated and hard to work with, so a lot of improvements that need to be made can't be made. It's also fundamentally insecure, as every window has access to both the contents and the input of any other window. Wayland is a modern replacement that focuses on security and expandability, and basically everything is working on switching to it. There are growing pains, but it's constantly improving, and most distros use it by default now.