Wayland is a display protocall. What does that mean? Well, it describes how a program on linux tha displays graphics to a user should work. Examples of programs which do this include kwin, mutter, and sway. It's why you see stuff on your monitor and why when you open an app, it's in a window, which can be moved.
All that along with a good amount of supplementary tools, makes up wayland.
Currently we are transitioning from a 40 year old protocal called X11 which has many issues and few people willing to work on. Wayland offers a codebase without the spaghetti, more security, easier feature development, and to a point backwards compatibility.