There is a big difference between a horse who is With or “on” the aids, and one who reacts to them. Anyone can make a horse go, turn, or stop with the threat of a kick or a pull. Many horses are trapped between the riders legs and reins – they obey them, but they bounce between one aid to the next. In other words; they react.
A horse who is “on” the aids accepts them, feels safer and happier because of them, and comes to the leg, seat and hand, not avoid them. This horse is with you happily, and doesn’t require constant nagging or threat of harsher aids to respond. They are light, not heavy, but not so light they hide from your hands behind the bridle. They respond to your seat with energetic, relaxed movement – not stiff, hurried steps.
This horse is a safe horse to ride, because he is with your aids; and with you. A horse who moves away from your leg and has to be caught by your hand may be obedient, but only til something worries him more somewhere else.