This cartoon is based on the double meaning of click.
A click is a short, sharp sound.
"The door closed with a click."
"Turn the handle until you hear a click."
In informal English, click is used to describe what happens when you suddenly understand or remember something.
"When she started talking about Boston, it suddenly clicked where I had met her before."
"I had to read the report a couple of times before it clicked."
A seat belt fastens around someone travelling in a vehicle or aircraft and holds them in their seat if there is an accident.