Sol

In Twilight Princess, a Sol is an object that lights the Twilight Realm, similar to the sun of the World of Light. It is made of pure power and provides energy and life to the citizens of the Twilight Realm. The light of the Sols, later in the game, fuses with the Master Sword to create a "Light Sword."

The two Sols that need recovering are in the Palace of Twilight: one in the east wing, and one in the west. When Link tries to remove a Sol from either wing, Zant's Hand will pursue. If Zant's Hand manages to take the Sol from Link and travel back through the portal in the wall it uses to travel from room to room, Link will need go all the way back to the chamber where he found the Sol originally and try to escape with it again.

Bringing the two Sols to the plaza in front of the Palace of Twilight enables their light to be infused into the Master Sword. Placing a Sol on special pedestals can cause platforms and steps to appear. Bringing a Sol (or the light-infused Master Sword) near to a cursed Twili will cause the Twili to revert to its original form.

Trivia

 * The word "Sol" originates from the Latin word for the Sun. Sol and its derivatives are are still used in a number of modern languages.
 * In Norse Mythology, Sól is the goddess of the Sun.