Feed-forward control usually means measuring disturbances that are direct inputs to the plant or process, and feeding these to the plant input (i.e. earlier in the forward path) through a feed-forward controller. The feed-forward controller TF is, ideally, the inverse of the process TF (or at least the part of the process that's affected by the disturbance), and the resultant signal is negated and added to the plant input signal, thereby cancelling the disturbance affects predictively.
TF inverses are sometimes awkward to implement practically, and an approximate realisation is often necessary.
Feed-forward control is largely independent of, say, a PID controller in the forward path, so design of the PID controller is not affected by the inclusion of feed-forward control.