Horses make many communicative chewing gestures with their mouths. They will begin to lick and chew, sometimes opening their mouths, while at other times keeping the jaw closed. Chewing gestures like this show that they are relaxed. It also means that they are herbivores and if they are in a place to eat, they feel safe and we are not doing any harm.
You may have observed this gesture when young horses make this motion out of respect for their elders – known as clacking or snapping. It’s a sign of submission. It’s also a sign that your horse is digesting and understanding information that he has been faced with.