So this is sheet music for trombone, you can tell by the slide position suggestions above certain notes. I assume they expect the player to have an F attachment, because 0 position is F, however the note is G, so they would need to press down the trigger to hit it (I think , it's been a moment since I've played a horn with an F attachment). Some of those symbols I'm not as familiar with.
Maybe the joke is they play the trombone, which intrinsically is a joke of an instrument.