An Automatic Activation Device is a small electronic unit fitted inside a parachute container that monitors altitude and rate of descent. If a jumper is still descending rapidly below a preset altitude, it releases the reserve parachute without any action from the jumper.
What it is for
An AAD is a last-resort backup, not a primary system. It exists for the situations where a jumper cannot deploy for themselves — unconsciousness after a collision, or losing altitude awareness.
Cypres, Vigil and Argus are among the widely known names; skydivers often say 'Cypres' generically the way people say 'Hoover' for a vacuum cleaner.
Why it is not a substitute for anything
An AAD firing means something has already gone seriously wrong. It is not part of a normal jump and is never a reason to deploy lower or pay less attention to altitude.
AADs also have service lives and require periodic maintenance by the manufacturer, and they can be affected by unusual descent profiles. Their handling and settings are matters for your instructor and rigger, not a web page.
When one is required
USPA Basic Safety Requirements mandate AADs for students, and many drop zones require them of everyone regardless of experience. Rental student gear will have one fitted.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.