A new feature introduced in Pokémon X/Y is the Mega Evolution. Unlike regular Evolution, Mega Evolution occurs only during battle. Once the battle ends, the Mega-Evolved Pokémon will return to its usual state.
Mega Evolution is not possible for all Pokémon. Only certain Pokémon, under certain conditions, can achieve Mega Evolution. One of those conditions is a special item called a Mega Stone. These special stones exist for every Pokémon that can Mega Evolve, and each is named for the Pokémon to which it grants power.
To use a Mega Stone, give it to your Pokémon to hold. Pokémon capable of Mega Evolution will gain access to their potential only if they first carry a Mega Stone into battle. Other things are needed for Mega Evolution to succeed.
Another item necessary for the Mega Evolution is the Mega Ring. When a Pokémon is holding its Mega Stone, the Mega Ring and Mega Stone resonate, triggering Mega Evolution.