Co-opted from the first episode of television show "Daria", it refers to a common courtship ritual amongst teenagers, in chronological order. "Movie", for going to see a film; "Burger", for having a bite to eat; and "Backseat" for the sexual activities that take place in the back of a car.
"They've been going out for a while and he's been getting upset because other guys have been asking her out. And she saying she can't help it if she's attractive and popular. And besides, nobody ever said they were going steady, and if he does want to go steady, he's got to do a lot more than just movie-burger-backseat, movie-burger-backseat, because there are plenty of guys with bigger backseats waiting to take her some place nice!"