Andúril is the sword reforged from the shards of Narsil for Aragorn.
Narsil was the sword of Elendil, the leader of the Númenórean exiles who founded Arnor and Gondor. During the final battle of the Second Age, Elendil was killed, and the sword broke beneath him. Isildur used the broken blade to cut the One Ring from Sauron’s hand.
The shards of Narsil were preserved by the heirs of Isildur. In Rivendell, they were reforged into Andúril before Aragorn’s journey into the wider war. The name is commonly translated as Flame of the West.
Andúril symbolizes Aragorn’s royal ancestry and his acceptance of the responsibilities connected with it. It is not the same weapon as Glamdring, Gandalf’s sword, or Sting, Bilbo and Frodo’s small blade.