Mega Evolution is a mechanic introduced in Pokémon X and Y, which further increases the abilities of specific Pokémon. If the player character possesses an item called the Key Stone and a Pokémon is holding a Mega Stone that corresponds to its species, that Pokémon will be able to Mega Evolve during battle.
In Pokémon X and Y, Mega Evolution is a feature that boosts the power of certain Pokémon. To use it, the player must have a Key Stone and a Mega Stone matching their chosen Pokémon's species. During battles, this combination allows the Pokémon to Mega Evolve.