1) The question that arose mid-ninties in response to the gang-related question "Where you from?", the purpose being a greeting that identifies the asker as a civilian and confers a subtle assumption that the person being asked is also a civilian or is not gang-related anymore.
2) A deviation of Standard English that asks about location.
"No, it ain't 'where you're from', you know it's 'Where you at?'" - Lighter Shade of Brown