What does " You want a piece of me" mean?

This's a scene of the movie , 'The school of rock"
The children's rock band goes to the place for audition.
Two Adults are saying to each other seeing some elementary students like that below way.
 ------The lines go:
-What's up, dude?
Check out these little kids, man.
- Hey, shut up.
- Oh, sorry.
Thought this was Battle of the Bands, not the Mickey Mouse Club.
- Hey, You want a piece of me?
- Look at this guy.
-Freddy! Come on, man.
We gotta focus up.
We gotta nail this audition.
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