The exp share item is found on Route 15 and is given to the player by Professor Oak's aid. The exp share grants the Pokémon holding it half the experience points gained from defeating a Pokémon, regardless if it actually participated in battle or not. This helps the player level-up Pokémon who are too weak to fight in battles.
Although exp share is an item that a Pokémon holds in “Pokémon FireRed,” it was changed to a key item in “Pokémon X” and “Pokémon Y” for the 3DS. This means that the Pokémon no longer has to hold the item to gain its benefits. This is helpful because all of the Pokémon in the player's active party gain the benefits of exp share.
The player may choose to turn off exp share at any time in “Pokémon X” and “Pokémon Y.” If a Pokémon actually participates in a battle, however, it gets 100 percent of the exp. This makes it more beneficial to actually use a Pokémon in battle when possible.
According to Nintendo Everything, Game Freak, the company responsible for developing the Pokémon games, said that the choice to make exp share a key item in “Pokémon X” and “Pokémon Y” came about because the developers wanted players to try raising lots of Pokémon. This is something they felt was more likely to happen if the player had an easy way to level-up his Pokémon.