A punchline is usually known in Comedy but it is also used in Rap.
Punchlines started in battle rap. Rappers would diss the opponent by setting up a line and finishing it with their “punch” line, which would be a witty insult.
A punchline is the witty follow up line to a previous line of lyrics. They can be funny similes, word play, puns, double entendres, or metaphors. For example Jay-Z is a great punchline rapper.
Example of a punchline:
“This shit is eternal, I rock the heavens well
Even if they won't let me in heaven, I raise hell, 'till it's heaven”
-Jay-Z, 7 Minute Freestyle.
A punchline is typically in order of two lines but they can be set up for more. A good punchline is one that impresses the listener or takes time to catch, or just plain really clever. A good punchline would be considered “bars”.
Punchlines are just a lot of punchlines.
Read: What does “Rhythm” mean?
Read: What does “Sauce” mean?