This method causes its connection to be authenticated. If authentication is enabled for the database server (it's not, by default), you need to log in before the database will allow you to do anything.
In general, you should use the authenticate built into Mongo::__construct() in preference to this method. If you authenticate on connection and the connection drops and reconnects during your session, you'll be reauthenticated. If you manually authenticated using this method and the connection drops, you'll have to call this method again once you're reconnected.