The forward movement is the basis for the development of impulsion in dressage. The horse should be responsive to the driving aids and move forward willingly. Nevertheless, it is not the only thing to pay attention to:
First: Forward is not only about speed but activity, especially from the hindquarters. Don't mix impulsion and `running.
Second: the movement should also be upwards: your horse should move `uphill´. The weight is shifted to the hind legs and they should step actively under the center of gravity.