X Window System (X Protocol)
The X Window System (also known as "X11", "X", etc.) is a display protocol that provides a window system on the display for remotely handling the UNIX desktop environment (GUI).
As for how you would use it, for example, you could remotely log in from your computer to another UNIX computer on the network and use that computer to do your work.
In the world of the X Window System, individual applications and displays do not have to be on the same terminal. Each client and server communicate using a TCP/IP-based protocol called the X Protocol.
From the user's point of view, the terminal operated by the user is the server side, and the remote computer is the client, which is the opposite of a typical client/server system.
The X Window System is hardware- and vendor-independent, so it can be used not only on UNIX and Linux OSs, but also on Windows OSs.
The X Window System was popular because it was cheap to display and process GUIs, especially in the days when UNIX machines were very expensive.