1) Less ghetto version of holla back, meaning to respond to a person at an unspecified later time.
2) To return to your country roots after a period of city living, as in Lost Trailers' song "Holler Back" ("holler" is a country term for a valley, a bastardized version of "hollow").
"If you wanna go on back to the holler, holler back!" -Lost Trailers
After the fifth time Judd had to blow a homeless guy for coke to keep him going through an all-nighter at his terrible I-banking job in New York, he couldn't take it anymore. The next day he packed up his stuff, took a dump on his boss's desk, and hollered back to become a farmer outside his hometown of West Shitville, Oklahoma.