What does to pick on somebody mean?

The meaning of the Word 'pick' is...

pick1    [pik] Show IPA

verb (used with object)1. to choose or select from among a group: to pick a contestant from the audience.

2. to seek and find occasion for; provoke: to pick a fight.

3. to attempt to find; seek out: to pick flaws in an argument.

So when someone is being picked on, they have been singled out by others to be bullied, to be harassed. They have some sort of negativity aimed towards them personally.

I hope this was helpful.