X11 forwarding is a technique used for starting remote application, but those applications are displayed to the user’s local machine.
X11 is an X Window System which allows its users to run software on a UNIX/Linux server in a Windows-like way. X11 forwarding is a secure way to run it. It is done by forwarding the X11 packets through the user’s SSH connection which sets the display environment variable automatically for the user. When the display environment variable is set manually, it is considered risky.