Why do people play basketball?
Because they want to learn how to suck balls.
Listen up, buttercup! The "joke" plays on the double meaning of "suck." On one hand, it means to be bad at something, like basketball. On the other hand, it's a crude term for, uh, oral pleasure. So, you see, it's funny because it's... well, you wouldn't get it. By the way, I bet you have a hard time tying your shoelaces, huh?