Tear of Light

The Tears of Light are light-infused magical teardrops that have the power, when collectively held, to energize Spirits of Light, as in Twilight Princess. They are also used to energize the sword, as in Spirit Tracks, the latter of which will be given the power to stun and possess Phantoms with Zelda's spirit. Tears of Light are non-equippable objects, meaning that upon their collection, they automatically perform their function whenever they are near something that requires their power.

Twilight Princess
In Twilight Princess, the Tears of Light, when collected within the Vessel of Light, are needed to revive the greatly depleted Spirits of Light, and return the land to its light-abundant state, ridding it of the twilight that had sapped the land of life. The Tears of Light are the key to not only awakening the provincial Spirit, but to lift the blanket of Twilight from the certain province in question so that Link can regularly traverse it while in human form. Link can only collect the tears in wolf form though, because they are manifested within Shadow Insects that can only be detected upon the activation of his Wolf Senses. After a certain Shadow Insect is defeated, it will reveal the Tear of Light within, and Link will be able to capture it in its respective container, the Vessel of Light. Commonly, these Shadow Insects exist in pairs, making it easier for Link to locate the Tears, and to aid in his search, each Spirit marks the location of each insect-infested Tear on Link's map subscreen, as well. As the game progresses, the number of Tears Link must collect increases, as does the distances between their locations and time spent collecting them. Link must return to the Spirit Spring after filling the Vessel of Light with Tears so that the spirit can be awakened and the cloak of twilight lifted.

Tears of Light can be found in three areas: the Faron Province, the Eldin Province, and in the Lanayru Province. However, Tears can only be found in one province at a time: the Spirit that requires them only marks the tear-holding insects on the map that reside within its province.

Spirit Tracks
In Spirit Tracks, each section of the Tower of Spirits features three Tears of Light that must be collected to power up Link's sword. Once all three Tears have been obtained, Link can strike a Phantom in the back, which stuns it; Princess Zelda's spirit can possess the Phantom while it is in this state. After Link obtains the Lokomo Sword, he no longer needs to use the Tears of Light -- the sword itself is infused with divine energy, and can allow Zelda to possess such Phantoms in the same way this previous sword did.

In the Dark Realm, Tears of Light appear on tracks. If Link passes through one on his train, it gets temporary invincibility, as well as a significant speed boost whenever Link blows the whistle. With this upgrade, it is then possible to crash into Armored Trains without dying and in turn destroying them.

Trivia

 * As said before, Tears of Light give the Spirit Train invincibility and increases his speed when used in the Dark Realm. They have the same use as Stars in the Mario series (not to be confused with Power Stars).