Two jumper cables walk into a bar...

Two jumper cables walk into a bar. The bartender says, "You guys'd better not start anything in here..."

This is a pun. To "start something" can mean to cause a machine to turn on (jumper cables are used to start cars that won't start), but it can also mean to cause a fight to break out, or to cause trouble. The bartender in this joke leaves it ambiguous as to which of these possible meanings he means.