What does nMotorPIDSpeedCtrl do?

Theoretically, you can have two options.
1. apply the required amount of power on the motors.
2. ask for the required motor speed

In the first case the electronics send the required power to the motor that will turn as it can (speed of rotation will depend on the load and the status of the batteries).

In the second case the computer checks if the motor turns the required speed and uses a fine tuning mechanism to balance the load (and battery status).