I'm guessing it's kinda like: you've got a couple people sitting there wanting to play with Legos and only so much room to play. They don't have direct access to the play area because of security reasons so they have to ask someone to place the Legos for them. Wayland, X11, and Xorg are all different people they can talk to to place the Legos in a way where no one is fighting for space.
So basically it's a new way for programs to negotiate who has what part of the screen?
I'm guessing Wayland is either more feature rich or lighter on resources and that's why it's a big deal?