The term refers to when a male courts or "Holla"s (holler meaning speak to) at a female, this could be a proposition for sex or sexual activities. Often the male will have many girls in which he holla's at or calls back again for more activities of the previously requested nature. At times he will holla back at her again, hence the term Hollaback girl. Otherwise know as HO, which is of course a shortened form of Hooker.
Gwen did not invent the term. The Rapper Fabolous invented it in his song "Hollaback Youngin WooWoo"
"I think I might Hollaback at Rochele, She had that Bomb..."
"Look boy don't you be comin round here trying to holla back at me again after you hit teisha and shakwandra,you triflin ass I told you once I aint no hollaback girl."