When the hobbits meet Aragorn in Bree, he uses the ranger alias Strider.
Frodo, Sam, Merry, and Pippin encounter him in the common room of the Prancing Pony. To the people of Bree and the Shire, he is a mysterious wandering Ranger known as Strider. The name reflects his rough, travel-worn reputation rather than revealing his royal identity.
Strider is not Aragorn's true name. He is Aragorn son of Arathorn, heir to Isildur and the future king of Gondor and Arnor. Gandalf's letter helps Frodo trust him, and Strider guides the hobbits away from Bree after the Black Riders discover their trail.
Readers sometimes confuse Strider with Aragorn's other names. Estel was the name used for him during his youth in Rivendell, while Thorongil was an alias he used during earlier service in Rohan and Gondor. Elessar became his royal title after he reclaimed the throne.