Engaging in a specific activity that you and your friends do a lot (considering "nigga" can be used to refer to someone as a close friend or "homie"). Generally to be shouted or yelled as you are entering a vehicle that will be taking to that activity.
Say, going to the mall.
Though, not to be said around black people.
Joe: Let's go to the mall and chill guys!
Everyone else: YEAH! IT'S NIGGA TIME!