The Light Stone is a key item in Pokémon Black that contains Reshiram after it lost its physical form. In the final battle against N in his castle, the player will battle Reshiram and will get the option to put it directly in their party for the following battle against N. Its Pokémon White counterpart is the Dark Stone.
Lenora will give the stone to the player in front of the Nacrene City Museum/Gym after he/she has defeated Team Plasma at Relic Castle in the Desert Resort.