Guest User is the meta-user whose layout is presented as the (unmodifiable) layout of the unauthenticated user. A portal maintainer "logs in as guest" to adjust the "guest layout". Changes to the guest user's layout are reflected in the unauthenticated view on the portal after the portal is next restarted, since the guest layout is aggressively cached.
If you're using an external authentication mechanism, such as LDAP, then maybe you have an external user named guest with a pasword. But you probably don't. So you need uPortal to provide a local mechanism to authenticate the guest user.
Out of the box, the guest user does not have a local password. There are two basic ways to configure the guest user with a password. One way is with an Ant task (accomplishing this task from the command line.) Another way is via an administrative channel.
Configuring a guest user password via the administrative channel is a two step process. First, create a user with username "guest". Second, set the password for that user.