They're calling the promotion, "Delay Your Way." They way it works is, if you're traveling at any airport in the US, you can enter your delayed flight information in the "Delay Your Way" screen of the BK app and, once its confirmed, get a digital coupon good for a free Impossible Whopper that's good at any participating Burger King restaurant nationwide. The coupon is good through January 6, 2020.
The Impossible Whopper is like a regular Whopper but with a plant-based Impossible burger patty instead of one made with beef.