It's Flava Flav's Public Enemy catchphrase. However, anyone can say it, as long as you mean it. It is one of the most positive things you can say. It's the best, the shizznit!

Examples:

Chuck D - Somebody in the house say yeah. If you really like to rock the funky beats, somebody in the house say HELL YEAH.

Flava Flav - YEEEEEEEEEEEEEEEEAH BOYYYYYYYY

or

Friend - You want to head to the club?
You - YEAH BOY