1) Another term to refer to something that's cool, awesome, sweet, tight, etc.
2) A totally kick-ass person.
Person 1- Hey man check out my new kicks I just got.
Person 2- Dude. Those are boss. Where'd you get 'em?
Person 1- Payless!
Person 2- BOSS
Person 1- Yo did you hear that new Lil Wayne song?
Person 2- Yeah bro I love that guy. Such a boss.