Z-Moves are battle element introduced in Pokémon Sun and Moon. They are ultimate moves of certain types.
Z-Moves require 3 things. The first is the Z-Ring, which goes around the player's arm and holds a Z-Crystal. Z-Crystals also need to be held by the Pokémon so they can resonate. The Pokémon also needs a move of the same type as the Z-Crystal. Once these conditions have been met, the trainer can activate a Z-Move once per battle.