Metagross's hand glows white or a silhouette of a golden meteor appears around its hand and it punches the opponent. When the attack makes contact, multicolored or golden meteor-shaped or star particles come out of the white light on Metagross's hand.
The Ability Iron Fist (which boosts the power of punching moves by 20%) affects Meteor Mash. This is because the Japanese name of Meteor Mash is Comet Punch (which was already taken by a Generation I move).
Despite this, no Pokémon with Iron Fist as an Ability can learn Meteor Mash. However, many Pokémon can still use this combination legally by using an Ability-copying technique such as Skill Swap or Trace and using the move either naturally, or through moves such as Metronome or Mimic.