Light Ball is a held item that doubles both the Special Attack and Attack stats when held by a Pikachu. Prior to Generation IV, only the Special Attack stat was boosted.
In Generation III and beyond, if a female Pikachu is allowed to breed while holding a Light Ball, the Pichu born will learn Volt Tackle. It is obtainable from a wild Pikachu, though there is only a 5% chance of it being held.
A female Pikachu holding a Light Ball in Pokémon Battle Revolution is one of the Mystery Gift gifts.
Locations
Ruby/Sapphire/Emerald: Held by Wild Pikachu
Omega Ruby and Alpha Sapphire: It can be found on Route 120.