Horses (and other animals in training) are problem-solving agents. To teach it something, you pose problems to it that it can figure out. Especially for dogs,

Horses (and other animals in training) are problem-solving agents. To teach it something, you pose problems to it that it can figure out. Especially for dogs, you pose a problem in the form of "How do I get a reward from this Hooman?" And by adjusting the difficulty of the required solution a dog understands what you want.