Young horses generally do not bite out of maliciousness, but rather playfulness. Horses play with each other with their teeth and hooves, and young horses have not yet learned that playing with humans similarly is not on the agenda. Youngsters need to learn where their space ends and yours begins, and that entry into your space is by invitation only. If the horse is not in your space, he is not in a position to bite you.
Positive reinforcement training is an effective tool to train this and similar ground manners. It teaches your horse that interacting with humans is good news, making the transition to under-saddle easy and natural. It is hard to mess up even for non-professionals. And, it is fun.
Positive reinforcement simply means that behaviours that are followed by a reward are strengthened – i.e. they are likely to reoccur (for more details see Horse Sport article here). To teach my horse not to bite me, I reward him for not biting me, and thus let him know that not biting is the behaviour I want to see again. There is a well-established myth that is a poor idea to give horses treats because it makes them nippy. However, treats do not make nippy horses unless the horse is reinforced for nipping. In fact, treats can teach your horse not to be nippy.