DA means district attorney. The relevance of this and urban terminology is it's connection with punishment for crimes. If you get caught by the 'Po (police for yall) selling drugs or whatnot, you will probably have to deal with a District Attorney. District Attorneys only practice law in district courts (imagine that). If you murder someone or get caught with enough coke to make a state, or even worse, a federal case, you won't be dealing with a DA, you'll be fucking with a whole new ballpark of lawyers and attorneys.
My homie Broham is in and out of court for possesion of narcotics and unlawful carrying of a concealed weapon. So, me and big 'Maal are about to show that fuckin DA how my strap works.