A Space Maintainer is an appliance that holds a space open between teeth. It is usually used in growing children, to make adequate room for a permanent tooth to erupt. Once the permanent tooth erupts, the Space Maintainer is removed.
A Space Maintainer can be fixed or removable; it can be made out of plastic or stainless steel, or a combination of both. There are several types of space maintainers; the one pictured above is only one example.