When working on a remote computer (which you connected with using rlogin or telnet, for example) and wishing to run code which would create a window on the screen of the workstation on which you are working, you must set the name of the display device on the remote system using the setenv DISPLAY command, and you also must permit access to your display device to the remote system using the xhost command.
For example, if you are remotely logged onto the UNIX computer crypt.phy.ornl.gov from pulsar.cs.utk.edu, and wish to create a window on pulsar, then in a window in which you are logged onto crypt, type
setenv DISPLAY pulsar.cs.utk.edu:0.0
to set the display on the remote system. Then, in a local window on pulsar, type
xhost + crypt.phy.ornl.gov
to permit access of the remote system to your screen.
Now, if you run the windowing application on the remote system, the display will be made to your screen.