debug allows you to troubleshoot problems or when your trying to optimize things like me
in admin/config.php add $debug=1; on a new line, then go to your forums and on the page you want to debug add: &showqueries=1 or &explain=1 to the url .. for example, if you want to see the queries being performed in forum home you're url should look like: /forums/index.php?s=&showqueries=1
don't forget to remove it from the config once your done.