Plain English Meaning
A system that moves the throttles for you to hold the speed or power setting you've asked for, so you don't have to keep adjusting them by hand.
Full Definition
An automatic flight system component that controls engine thrust by adjusting the throttles to maintain a commanded airspeed, Mach number, or thrust setting selected by the pilot or commanded by the flight management system.
Context Anchor
Seen in aircraft automation, flight management system, and instrument flying discussions where speed and engine power are managed by onboard .