Aragorn uses the alias Strider when he first meets the hobbits in Bree in The Lord of the Rings.
At the Prancing Pony, Frodo and his companions encounter a weather-beaten ranger known as Strider. His true identity is concealed at first because he is protecting the hobbits and operating among people who do not know his royal heritage.
Aragorn is later revealed as the heir of Isildur and a descendant of the kings of Númenor. He also uses other names during different stages of his life, including Estel in his youth and Thorongil during an earlier period of service. After the War of the Ring, he becomes King Elessar. These names refer to the same character, but Strider is the specific alias used in Bree.