Even more directly, the CGI parameters are a way for the browser to pass information to the web server so the server can process it and spit back a custom response. Usually this information is entered in a form on the page. So if you were searching for a flight, the url parameters might include the dates of travel and airports you enter.

There are other ways of passing this info to the webserver that don't involve user-viewable parameters in the url, so if you don't see them then that's what's going on.