HB stands for hot babe. However, it is used generically for any decent looking female. She doesn't necessarily have to be hot. Her hotness or lack thereof is given by the number after HB. It's the classic one through 10 scale. 7 is considered cute and doable. 8 is very cute. 9 is smoking hot. 10 is supermodel perfection. Below 7 can be doable. Just don't tell your friends lol. Finally, these appraisals are best performed sober lol.
So an HB8 is a very cute, bordering on hot female.
This system is not necessarily meant to objectify women, but to put the pickup attempt in context for the readers. If for example, a guy writes that he made out with some HB (and no number is included), the reader will wonder what she looked like.