How do I understand the phrase " closure" mean?

[english-learner]

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)

[trustM]

It means end, ending. She is over him and that's it, their relationship ended, closed.