A prom is a formal dance at the end of a school year. For many American high schools, the senior prom is a big deal. Kids get dressed up and then get embarrassed by parents who take too many photos.
Students going to a prom will sometimes rent limousines to get there, and dress in fancy gowns and tuxedos. The prom itself is generally a much fancier version of a school dance, often held in a hotel or a gussied-up school gymnasium. The word dates from the late nineteenth century, an American English shortening of promenade, which means "to stroll," but also "to dance in couples with joined hands."