It's an abbreviation for "Ass In Seat" or else. It started from the sitcom "Everybody Loves Raymond" where Ray tried to pull it on Debra, but he was inept to know that you never pull an AIS on your own wife, but on the kids, just to teach them a lesson about being on time. It is usually said when you want your passengers in the car, so you can drive at the right time.
Ross: Alright Mon, for the party tomorrow we're doing an AIS or else at 5:00 p.m.
Monica: Alright, agreed, AIS or else at 5 pm.