Found this description (Search: FeedForward PID controller) on the net.
In contrast to feedback control, feedforward control acts the moment a disturbance occurs, without having to wait for a deviation in process variable.
This enables a feedforward controller to quickly and directly cancel out the effect of a disturbance.
To do this, a feedforward controller produces its control action based on a measurement of the disturbance.
When used, feedforward control is almost always implemented as an add-on to feedback control.
The feedforward controller takes care of the major disturbance, and the feedback controller takes care of everything else that might cause the process variable to deviate from its set point.