PID - Proportional-integral-derivative
PID controller is a type of controller that is widely used for a variety of applications. The name comes from its use of the integral and derivative of the error term, as well as the error term itself. If only two of the three terms are used for a controller it is usually called PI (proportional-integral) or PD (proportional-derivative).
Ashes has a built-in PID controller that can be used for: Demanded pitch scheme (parameter under Rotor ), Pitch actuator mode (parameter under Rotor ), Generator (parameter under RNA ), Yaw scheme (parameter under RNA ). In all cases, the built-in PID controller is used when the option PID controller is chosen.
More about PID: Wikipedia.