An X server is the display server in the X11 graphical stack. It draws windows on screen, processes keyboard and mouse input, and mediates access between applications and the user session. In practice, many X11 programs connect to the X server over a local socket, and sometimes over a networked session, so it becomes a central trust boundary for desktop systems.
Security matters because flaws in the X server can affect the entire graphical environment, not just one app. A bug may let an attacker crash the session, read sensitive screen or input data, or in the worst case execute code with the privileges of the display service or logged-in user. Defenders reduce risk by keeping X.Org packages patched, limiting who can connect to the display, disabling unnecessary remote access, and migrating legacy X11 applications to safer compatibility layers such as Xwayland when possible.