A baby tooth usually falls out because there is a permanent tooth erupting underneath it. When this occurs naturally, the space vacated by the baby tooth is filled quickly by the adult tooth replacing it. If for some reason the baby tooth comes out early (accident, decay, etc.), then it is important that the space left by that tooth be maintained for the adult tooth. Otherwise, the teeth on each side of the opening may collapse into that space, and it may require orthodontic treatment to correct.
When a child loses a baby tooth early, we use special appliances called space maintainers made of metal or plastic to hold that space open. Children usually adjust very quickly to the space maintainers, and it requires only a little extra care by you and your child to keep clean.