The rider gives the horse instructions by means of the aids. There are two types: the natural aids - the rider's seat, hands, legs and voice; and the artificial aids - spurs, whips and some items of tack such as martingales.
The hands and legs operate in combination with each other and should be applied in a co-ordinated way. There is little point in using your legs to tell the horse to walk on if your hands are stiff and resistant to its moving.
You must be able to use each hand and leg independently. Your inside hand controls the horse's direction, your outside hand controls the pace, your inside leg creates impulsion, and your outside leg controls the horse's hindquarters so that they don't swing out.
To create impulsion you make use of the fact that a horse always moves away from the rider's leg.
If you apply pressure to the horse's sides by closing your lower legs against it, the horse will move forwards away from the pressure. If you apply pressure on one side only the horse will move away from that leg.
In order to control and make use of the energy created by your legs, you use your hands to contain the horse's forward movement, thereby creating an active pace in the horse. You need to have an independent seat in order to use your legs properly and be free of your hands, that is, not learning on them for balance.
You muct be aware of how you feel to the horse. If your contact is too strong or too light, it will let you know by carrying its head incorrectly, by chewing its bit, in the way it moves, and even by going faster to try...