The horse is rattling his harness bells, as if he is wondering if halting is a mistake. This poem is about a person driving a horse-drawn carriage through the woods on a winter evening. As a result, the horse is concerned for his master. Horses have an innate sense of when someone is afraid or unsure of themselves. They can read our body language and emotions, so they will often try to comfort us by doing what's called "mirroring" behavior. That means acting like something is wrong even when there isn't anything wrong. For example, if you are angry, horses might sidestep away from you or shake their heads in protest even if they aren't aware of why you are angry. This poem is about a man driving a carriage through the woods at night. Because it is winter, there is no way for him to know if the path he is taking is safe. So the horse prepares himself for whatever may happen by being ready to help his master if needed.