Stands for Underage Boobies. While it can be used to describe the breasts of an under-18 year old girl, it is usually used to describe a hot under-18 year old girl that usually provokes a look-but-don't-touch reaction. Alternately it can describe a number of teenage girls when used in a blanket description such as "many UB".
Damn, did you see that UB over there?"
"Hey Jim, a whole busload of UB came into work today for 'Take Your Daughters To Work Day'"