A Phial containing the light of the evening star (Eärendil), caught in the water that filled the Mirror of Galadriel. Galadriel's gift for Frodo.
From the top:
Varda Elbereth Gilthoniel, the Star Kindler, strews a lane of stars and milky light thru the sky, to herald and illuminate awakening elves at Cuiviénen.
Yavannah collects pools of this light to feed the Two Trees. They illuminate Aman, and cast a twilight into Middle-earth.
Fëanor constructs Silmarils with the light of the Trees. Melkor covets the them, and conspires with Ungoliant the Spider. She poisons the trees.
Melkor steals the Jewels, and starts the War of the Jewels.
Eärendil uses the light of a Silmaril to enter Aman and speak with one voice for the salvation of Eldar and Men.
The Valar hallow Eärendil's ship, Vingelot. They put and his Silmaril in the ship, and set it in the sky.
Humans descended from Beor, Fingolfin and Melian follow the light of the Silmaril - the Morning Star - to the island of Ak---, and found Numenor.
Almost two ages later, Galadriel must help Aragorn & Gandalf put down Sauron and found a new Numenorean dynasty. She collects the light of the Morning Star and gives it to Frodo. She gives him a symbol of the foundation of Numenor, to help him found its next incarnation.
On the eaves of Mordor, Samwise points out to Frodo "We've got a bit of that light in the Lady's Glass".
In the next chapter, Frodo and Samwise use the Light against Shelob, the daughter of the Ungoliant.