This quote is saying that horses are like a mirror. Whatever emotion someone puts into interacting with a horse, the horse will return the same emotion. So, if someone approaches a horse with kindness, respect, and love, the horse will show them the same kindness, respect, and love. This can be a good thing, because the horse’s behavior can tell us something about ourselves. In other words, we can use the horse to understand how we appear to other people and to reflect on our own attitudes and emotions.