Q: Why did the belt get arrested?
A: Because it held up a pair of pants!
Explanation: To hold up: this phrase has more than one meaning. You could take a magic wand in your hands and hold it up in the air like Harry Potter; a piece of tape can hold a piece of paper to the wall; a belt can hold up a pair of pants. In these examples one thing holds another thing in place.
To hold up a bank means to rob it, to steal money from the bank.
This joke is funny because of the multiple meanings of “to hold up.”
Here is Woody Allen trying to hold up a bank: