This is a traditional greeting originating in, and mostly heard in, the city of New Orleans. This expression arose decades ago among the large community of itinerant musicians ever passing through and sometimes settling in the New Orleans French Quarter. When one musician would pass another on the street, they'd often stop briefly and each would inquire as to where (in which music bar?) the other was currently playing. Thus, the question, "where are you at?" just means 'where are you playing?'. The phrase got contracted to 'Whereyat?' and became the same as saying 'Hi'.