Taking away something that the horse dislikes in order to reward a desired response. Giving the horse relief or escape when he makes the correct choice.
For example, removing leg pressure as soon as the horse yields sideways. Softening the pressure on the lead rope for a forward step when teaching a foal to lead. Quieting a clucking noise as a horse lengthens his stride on the lunge.
Can you see how timing is critical? As an experienced riding instructor having an eye on the ground helps until they develop a feel for it themselves.