Here is the context:
Rachel: (on phone) Ross, hi, it's Rachel. I'm just calling to say that um, everything's fine and I'm really happy for you and your cat who, by the way, I think you should name Michael. And, you know, ya see there I'm thinking of names so obviously, I am over you. I am over you and that, my friend, is what they call closure. (hangs up and tosses phone in the ice bucket)
It means end, ending. She is over him and that's it, their relationship ended, closed.