The Animus lets Desmond Miles experience the memories of his ancestors in Assassin's Creed.
In the series, the Animus is a machine that reads genetic memories encoded in a person's DNA. Desmond uses it to relive the lives of ancestors such as Altaïr Ibn-La'Ahad and Ezio Auditore. These historical sequences form the main playable settings, while Desmond's modern-day experiences connect them to the wider story.
The device is fictional, and the games use it to explain how a modern character can participate in events from centuries earlier. The technology also supports the series' recurring ideas about memory, identity, control, and inherited history.
The Helix is a separate entertainment and research platform associated with later games, especially Assassin's Creed Unity and Rogue. It is therefore an easy term to confuse with the Animus. The Animus is the foundational machine introduced at the beginning of the franchise and remains its best-known fictional technology.