The Yii Framework is very powerful and it provides a lot of functionality right from the pre-built webapp. One of the nice things that is already established for you as a developer is the Login authentication. While the default configuration simply sets it up to run against an array of hard coded usernames and passwords, the Yii Blog Tutorial provides a how to in connecting that login authentication method to a database so you can run your logins against the registered users. Talk about sweet deal.
These features provide endless possibilities and unbelievable flexibility, but as current documentation does not give more than a few examples, it might be difficult to fully understand their internals and requirements.