Yo man, stand up.
*short person stands*
No, seriously man, stand up!
Yo man, stand up.
*short person stands*
No, seriously man, stand up!
Alright, listen up, you knucklehead. This joke plays on the double meaning of "stand up." The first guy is telling the other dude to literally stand up. But the second time, he's saying "do some stand-up comedy." Since the other guy is a shorty, the first interpretation is the most obvious. Get it now, or do I have to draw you a picture?