DISPLAY

You have a X Window server running on your machine, at hostname. You can run a client application on a remote machine, at rhostname, and view it on the local machine.

On the remote client:

export DISPLAY=hostname:0

(or in general hostname:D.S where S is the screen and D is the display.)

On the server:

xhost + rhostname
xhost -

Xhost is a very insecure mechanism. Use ssh -X to do this more easily and securely.