It's an expression meaning to telephone someone. If a guy tells a girl in passing to "holla back", he's looking to get with her. If the girl calls she is a "hollaback girl".
a holla back girl starts off as a booty call. the guy wants to "holla" aka.. he needs some ass and after that particular bootycall, he forgets all about their little "fling" until one day he wants another booty call from that same person. as in.. he wants to "holla back" therefore, you get "holla back girl"
ryan wants to mess around with someone. so he calls susie, and they messaround that night. the next day, ryan completely ignores susie. this goes on for about a week and a half. until one day ryan decides that he wants to mess around with susie again. so he calls her to "holla back" at her. but she refuses. thus proving that she is not a holla back girl.
A holla back girl is a hoe fo sho or a skank, slut, or whore.
You see when a gangsta see's a fine fee or an attractive woman walkin down the street he will holla at this female to see if she is a hoe fo sho. If this female is indeed a hoe fo sho then she will holla back making her a holla back girl.