The answer may be better illustrated as an example. If JP changed the port that Apache listened to for HTTP requests to port 8080 (because programs that bind to ports above 1024 don't have to be started as root), the URL for AO would look like this: http://www.antionline.com:8080/

The default port for HTTP requests is port 80, so if you visit: http://www.antionline.com:80/ you will be shown the AO homepage.