Superpower is an offensive Fighting-type move, which also lowers the user's Attack and Defense stats by one stage each.
As of 1.0.4, it can be taught for 4 Red Shards by the Physical Move Tutor. Prior to 1.0.4, it could be taught by the Ultimate Move Tutor for four of each shard.
Pokémon that learn Superpower
By leveling up