I'd expect that's the tied object method that gets called when you test a tied variable for definedness. Of course, I'm just speculating, but then, your question is vague too. In what context did you see this?
Its mentioned in perlfaq4 (or 'perldoc -q defined') but its not mentioned in any of the Tie::* modules documentation, nor in perltie...sounds like a doc patch might be called for...