What “Zeen” means
Zeen is a variant of seen and means "cool", "okay" or "understood". It can agree with something, or, tagged onto the end of a sentence, ask whether the other person understands.
Examples
Zeen man
Okay, understood.
Wi a lef early, zeen?
We're leaving early, you understand?
How to use it
Very casual and popular with younger speakers. As a tag question it seeks acknowledgement more than agreement.
Where usage varies
You'll also hear "Yuh zimi?", a squeezed form of "you see me?", meaning "do you get what I mean?".