Plain English Meaning
It is the box of sensors that tells the cockpit screens which way the aircraft is pointing and how it is tilted. Without it, the electronic attitude indicator and heading indicator would have nothing to show.
Full Definition
An AHRS unit is a solid-state electronic device that senses aircraft motion using accelerometers and gyroscopes (often supplemented by magnetometers) and outputs continuous attitude (pitch and roll) and heading information to the cockpit displays. In glass-cockpit aircraft, the AHRS unit replaces the older mechanical gyroscopic instruments that traditionally provided this information.
Context Anchor
Seen in glass-cockpit and instrument-flying discussions, especially when comparing electronic heading information with a traditional magnetic compass during .