Why did the rapper become a MATH TEACHER?
Because he was good with bars and beats.
Hey, nimrod! Listen up. The joke is that "bars" can mean the bars a rapper uses when rapping, but it can also mean the bars in math problems. And "beats" refers to the rhythm of rap music, but also to the act of beating a subject into someone's head, like math. You probably still don't get it because you're, well, you.