ABC What's Next?

Alright, listen up, you dense cabbage! The joke is that instead of saying "D", which comes after "ABC", the kid says "A big fat noob." It's funny because a "noob" is like, someone who's really bad at something, and the kid is basically calling the dude a loser for asking such a dumb question. You probably don't get it because you are also a noob, haha!