The SelfService interface is available for "unprivileged users" (see User for a definition) who have been given a password to login. You can configure RT to automatically generate a password for users by following the directions in the AutogeneratedPassword node.
Example: A department manager may be made CC on all tickets created by her staff so that she is be able to see what issues are currently affecting her department.
The SelfService interface is accessible by appending "SelfService" the end of your RT Web site URL. For example, if your RT Web interface is setup at http://support.mycompany.com, the SelfService interface would be at http://support.mycompany.com/SelfService/