IMU or inertial measuring unit is an electronic sensor composed of accelerometers, gyroscopes and magnetometers. It measures a device's velocity, orientation and gravitational forces.
In virtual and augmented reality, IMUs are used to perform rotational tracking for the HMDs and the Input Devices. It measures the rotational movements of the pitch, yaw, and roll. IMUs are responsible for 3 degrees of freedom of a 6DOF tracking system.