I can only imagine it means to pick off a berry from a plant, or pick [a person] off a list of candidates. Those are just two examples. I think you could also say 'to choose'.

Can you give a sentence in which you use it? It can mean several things.

Yes, as Amanda mentioned, more context is needed. Pick off can also mean to snipe someone (to shoot (kill) them from a long distance), although that doesn't sound very 'Friends'.

"Pick", I'd go for to be close to "choose" often. "pick off" seems more often used mean remove one form a group. There were more of them than us. We could not fight them openly. We decided to try to lure some away and pick them off one by one.