"Calling" the script means browsing to it from your favorite web browser. For example:
http://www.Your-Domain.com/YourChatP...stallation.php
Every time you click on a PHP link you are "calling" or "running" one or another PHP script parsing files, which, in turn, dynamically creates an HTML page for you. Just like when you did your original install of vB, unless of course you have an illegal copy. That's why php install files must be deleted after use, they can reveal far too much about your setup.