X11 Window System

X is an application that is used to manage input devices, like a mouse and a keyboard, and the output devices like displays connected to a system. Any user application can communicate with the display graphics interfaces using different service routines.

Official documentation is at:

https://www.x.org/wiki/Documentation/

In the context of running graphic applications on top of X, in previous releases you could either have X running by default on the Ubuntu root filesystem started by the lightdm or gdm3 service, or start it manually. In this release, lightdm or gdm3 is not enabled by default. You must start X server manually.