I think it's actually called "work offline", and yes, it is a Project
Server function, which I don't have where I currently work. It's
helpful for working on a schedule at home, or anywhere you can't be
connected to the server. It "checks out" the project on the server so
that no one else can access it read/write, and saves it to an obscure
directory on your system. Then you can work on it at your leisure. When
you are back connected to the server, you save it back to the server,
essentially "checking" it back in.