the URL you want to appear inside the facebook application. This module kid of creates an application for facebook (actually projects a page of your olphin site to facebook). So if you want it to show your memberlist for example , you'll point the backtrack towards www.yourdolphinsite.com/members.php
The actual meaning is creating facebook applications to promote your dolphin site through facebook.com.
The problem is there is no such FB application yet...youll have to create your own :/