Can someone explain?

Another word for the season of Autumn is Fall.

Thus, the DM is about to make a pun about Fall damage. The player of the bard notices the incoming pun, takes defensive action via casting a counter-pun spell, and thus prevents a table full of groaning and moaning, and possibly even laughter.

Laughter from a pun like this is a rare thing, it’s a charisma check of 19 with disadvantage.