From Bulbapedia, the community-driven Pokémon encyclopedia.
Jamming is a term used in Pokémon Contests and is a feature only in Generation III; it refers to when an appeal can lower the appeal points of the other Pokémon in the contest by the number of black hearts represented above the number of appeal points the move makes.
If not executed correctly it only says that the move messed up, and it still gains appeal points and any other extra, for example, if a move that affects all Pokémon after the user is played last. On the other hand, if the move backfires on the user, it causes them to lose appeal points.
Moves which can jam