The punch line is the last line of a joke or funny story that shows why it's funny. It's a metaphorical punch. It's when the point of the joke 'hits' you.

A punchline can also be used as a phrase that is your point of view. The important point you want to get across to people.

Making an audience laugh can be a form of dominance, and comedians sometimes use violent analogies to describe it. "I knocked 'em dead," "I killed 'em tonight." The "punch line" is the last line, the one that delivers the knockout punch.