This means that if a row is retrieved more than once, no matter how you retrieve it, you get the same object back.
The example on screen shows two completely different methods of retrieving an object.
If you run this without the identity map, the result will be false. If you are using an identity map, the result will be true.
By itself, an identity map is not very useful, but Sequel also uses it as a simple cache, so it can be a performance enhancer.