In Harry Potter and the Prisoner of Azkaban, Hermione’s time device is called a Time-Turner.
Hermione receives the Time-Turner from Professor McGonagall so she can attend several classes scheduled at the same time. The device resembles a small hourglass on a chain. Turning it allows its user to travel backward by a limited number of hours, making it possible for Hermione to fit more lessons into one day.
Near the film’s conclusion, Hermione and Harry use it for a much more serious purpose. They return to an earlier point in the evening to rescue Sirius Black and the hippogriff Buckbeak. Their actions create the appearance that events happened as already witnessed, rather than changing the past in a simple way.
The film does not present time travel as an unlimited solution to every problem. The Time-Turner’s use is tied to the events of that night, and later films do not make it a routine plot device. It is therefore distinct from a Pensieve, which stores and replays memories.