“detached” means you’re dealing with an ORM object that is not associated with a Session. The Session is the gateway to the relational database, so anytime you refer to attributes on the mapped object, the ORM will sometimes need to go back to the database to get the current value of that attribute. In general, you should only work with “attached” objects – “detached” is a temporary state used for caching and for moving objects between sessions.
See Quickie Intro to Object States, then probably read the rest of that document too ;).