Electronic speed controller
An electronic speed controller, or ESC, is a circuit that takes battery power and drives a brushless motor at whatever speed it is told to. It switches current through the motor's coils in the right sequence, many thousands of times per second. On a drone, each motor has its own ESC, and the flight controller sends speed commands to all of them.
Example: On a quadcopter you can usually see four small ESC boards, one wired between the battery and each of the four motors.