"Nulled" software means that paid software, such as vBulletin, Invision Power Board, WHMCS, etc, have been modified by a user to no longer "phone home." Software reaches out to a server (like Adobe products connect to activate.adobe.com) and make sure that everything is in order.
Nulled software has been modified not to do this, so it usually cannot be tracked by the company that owns the code.It's basically like making your computer not be able to connect to activate.adobe.com so that it can't check if your license key for a product is valid.