A piece of urban/ghetto slang with many definitions. Here are some of the more common ones, as determined by many Google searches.
1. To call back later, to get back to someone.
2. An angry response to improper advances / harassment.
3. Someone who will make a lot of noise but not act. All talk, no fight.
4. {Holla back girl} A girl who will let anyone (especially boys) walk all over her. A girl who is at a boy's (or many boys') beck and call.
5. {Holla back boy} A boy who won't make advances on a girl, unless she makes one first. A boy at a girl's beck and call.
The http://www.urbandictionary.com was very useful in creating this node, and has other definitions and variations on definitions that I have not included here.