Flight controller
A flight controller is the small onboard computer that keeps a drone stable and steers it. It reads motion sensors such as gyroscopes and accelerometers hundreds or thousands of times per second and adjusts each motor's speed to hold the aircraft level and follow pilot commands. Many also connect to GPS so the drone can hold position or fly home on its own.
Example: If you nudge a hovering quadcopter by hand, its flight controller notices the tilt and speeds up the low-side motors to level it again.