Wayland is a display server protocol and implementation designed for Unix-like operating systems. It is an integral part of the GUI of an operating system. First released in September 2008, it is open-source and compatible with Linux, FreeBSD, and DragonFly BSD. It is intended as a replacement for the X Window System (X11).
Compared to X11
Wayland tightly integrates some of the modular and loosely-connected components of X11, such as the compositor. These design choices improve performance, and make Wayland easier to develop and maintain.
However, the X Window System still has many advantages over Wayland. As of 2019, the majority of video games and graphics-intensive applications for Linux are still written for X11. Also, many closed-source graphics drivers, such as those for NVIDIA GPUs, do not yet offer complete support for Wayland.
Wayland is the default display server in Ubuntu version 17.10. In Ubuntu versions 18.04 and 18.10, X11 is the default display server, and Wayland is offered as an option.
More information about Wayland is at https://wayland.freedesktop.org.