Holla is a well known term in the "gangster" world meaning Hello or How Do You Do? Front refers to a person or "homies" house or "hang out" spot. Put two together and it simplys means dropping by your friends(homies) house(hang out spot) just to say hello, maybe have a beer and fancy a game of criquette on the tube (television).
Hey homie want to go to my house and hang out? Holla Front!