I actually like the beat, even though the lyrics suck. Definitely not a fan of her's.
And a "hollaback girl" is a girl who lets her guy do anything he wants to her without fighting back or resisting in any way. Basically a passive wimp.
Or it could mean you second choice girl...the backup girl that you ask out or call because the first girl you tried turned you down. The actual name "hollaback girl" makes this make sense, because you're hollering back only because you couldn't get someone else.
But with the feminist crap she pulled in those (horrible) lyrics, I'm guessing it's the first.
Edit: Wow, I wasted my 4000th post on this uninformative crap.