The user eats its held Berry, and raises its Defense stat by two stages in addition to whatever effect that Berry might normally have. The user can successfully eat its Berry even in the presence of Unnerve or Magic Room.
If the user is not holding a Berry, Stuff Cheeks cannot be selected from the move menu, and if the user has no other moves with PP left, it will be forced to use Struggle. If a Pokémon would use Stuff Cheeks, but is not holding a Berry when it would use the move (e.g. if its held Berry had been removed by Knock Off), Stuff Cheeks fails.