A black person's excuse for being ignorant or doing ignorant shit. The person who says that they are "keepin' it real" usually claims to be denying pop culture and sticking with their own thing.
Chris Rock had a good description:
Rock: Nothing makes a nigger happier than not knowing the answer to your question:
(first person):"What's the capital of Zaire?"
(nigger):"Shit, I don't know that!....Keepin' it REAL!"
Rock: Yeah, real DUMB!