Joke:

“So a man was sitting alone at a bar and he kept hearing this whispering that said “hey nice shoes” he looked around and nobody was there he heard “You look nice today” he looked around nobody was there. Then he finally asked the bartender “Am I hearing anything?” and the bartender said, “it’s the peanuts. They’re complementary”

My informant loves to tell this joke because it’s clean and it’s a pun.

Puns are a way of playing with language in a way that messes with how similar words can mean very different things.  In this joke, complementary means both free and giving complements.  I’ve heard lots of jokes where the bartender responds with a pun. It is a popular genre where the idea is that the bartender is the one who knows what is going on while the patron is confused or, a similar one, is where the bartender acts as the frame for the joke.  This may be because a bartender is expected to be the one in control of a bar. They are the ones who control the intake of alcohol and are there to interact with the patrons.