The idea is that the user requests for example <a href="http://www.site.com/pretty_file.htm">http://www.site.com/pretty_file.htm</a> (this link is indexed by search engines) and in reality browser shows the content of say <a href="http://www.site.com/index.aspx?id=123">http://www.site.com/index.aspx?id=123</a> (that is real physical file on your server).