An IMU is a device which is used to measure positional and vector deviations from a known start point/vector. They are the core of Inertial Navigation Systems, some autopilots, and more fun gear. At its most basic, an IMU consists of three accelerometers, which allows it to keep track of any changes in velocity in three dimensions, and an accurate chronometer. Sometimes they use physical instruments, and other times solid-state (such as the ring laser gyro).