Exp Share is an item obtained by fainting or capturing wild Pokémon. It splits the experience points that the user earned in Pokémon battles.
To use Exp Share, drag the item to the item slot in your Pokémon by clicking their name above. This way the experience will be split equally between the two Pokémon.
*Keep in mind that the Pokémon that you give the Exp Share gets the shared experience points.
Note: The Drop Rate of this item is still not clearly known