Aragorn is the heir of Isildur who becomes king in The Lord of the Rings.
Aragorn first appears under the name Strider, a Ranger of the North. He is secretly descended from Isildur, the founder of the royal line of Gondor and Arnor, and is therefore the rightful heir to the throne of Gondor.
During the War of the Ring, Aragorn helps protect Frodo and supports the military resistance against Sauron. He receives the reforged sword Andúril, made from the shards of Narsil, Isildur’s sword.
After Sauron’s defeat, Aragorn becomes King Elessar and rules the Reunited Kingdom. His marriage to Arwen joins the royal line with one of the great Elven families. Aragorn is not the same character as Boromir or Faramir, both sons of Denethor, the Steward of Gondor.