This is High Power 12V/24V/36V. 40A PWM DC Reversible Motor Controller.  A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.

The 40A PWM controller is equipped with High Power automotive relays, a Knob to adjust the output operating Voltage, a toggle rocker switch to change motor rotation forward-reverse direction, and a 3-digit seven-segment LED display to show the selected Voltage