Anything that's promotional is meant to sell or advertise something. A promotional book tour is a trip an author takes to various bookstores hoping to sell more copies of her new book.
Items that are handed out at an event or trade show, especially if they're marked with a logo or brand name, are promotional materials. They're free goodies, but they're also used to advertise a certain company or product. Your promotional travel mug probably has some company slogan printed on it, for example. A rock band's promotional appearance at a music store is a quick stop meant to advertise (or promote) a new album or concert tour.