In Assassin's Creed, the Animus allows modern people to relive their ancestors' genetic memories.
The Animus is a fictional machine central to the series' structure. A user lies inside it while the device interprets inherited memories and renders them as an interactive simulation. This framework allows the games to move between modern-day scenes and historical periods without treating the historical sections as ordinary time travel.
Desmond Miles is the first major modern protagonist shown using the machine. Abstergo Industries initially uses him to investigate his ancestry, while the Assassin Brotherhood seeks to exploit the same technology for its own purposes.
The series later introduces related systems, including the Helix entertainment platform. The Helix is a separate consumer-facing technology, so it should not be confused with the Animus itself. Likewise, Pieces of Eden are ancient artifacts, not memory-reading machines.