All you need to do (this is a MUST) is provide implementation to the hashCode() method in the Primary Key class. You would retrun an integer value for the hashCode of the primary-key. The hashCode() method is one of two methods you MUST implement in the Primary Key cassl of an Entity Bean. The other method is the equals() method.
The short answer is that it's a unique identifier for the instance. For more information on hashing and it's purpose, look at java.util.HashMap - or lookup HashMap in a good data structures book