Space maintainers are appliances made to custom fit your child's mouth to maintain the space intended for the permanent tooth when it decides to come in. They do this by "holding open" the empty space left by a lost tooth by preventing movement in the remaining teeth until the permanent tooth takes its natural position in the child's mouth.
This treatment is much more affordable and much easier on your child than to move the t eeth back later with orthodontic procedures. Think of space maintainers as insurance against braces.