Aragorn uses the assumed name Strider when he first appears in The Lord of the Rings.
The name is used in Bree, where the hobbits meet him at the Prancing Pony. At that point, Frodo and his companions know him as a mysterious ranger rather than as the heir of Isildur and the future king of Gondor.
Strider is a translation of the nickname Tolkien gives him in English. Aragorn also has other names and titles, including Elessar and Thorongil, but those belong to different parts of his biography and should not be confused with his initial identity in Bree.
The gradual revelation of Aragorn’s lineage is one of the central character developments in the trilogy. His ranger identity connects him to the wild lands of Eriador, while his ancestry connects him to the kingdoms of Men and the history of the One Ring.