Plain English Meaning
Small sensors built into the AHRS that feel how the aircraft is turning and how it is being pushed around. They sense the motion from inside the aircraft, without looking at the ground, the sky, or any signal from outside.
Full Definition
Devices that detect motion of the aircraft itself — specifically rotation rates and linear accelerations — without reference to anything outside the aircraft. In an Attitude and Heading Reference System (AHRS), inertial sensors typically consist of three rate gyros (or gyro equivalents such as MEMS or ring-laser units) measuring pitch, roll, and yaw rates, and three accelerometers measuring acceleration along the aircraft's longitudinal, lateral, and vertical axes.