Can docs explain what does the orthodontist mean by "space maintainer"?

A device...: .. To maintain space. It does exactly what it is called. A primary tooth that is lost prematurely can allow secondary teeth to drift into the space before the associated adult tooth can erupt. The maintainer helps keep those teeth from shifting and helps prevent more complex orthodontic care. It is well worth the investment.

Read Below: A space maintainer is a dental appliance that literally hold space. The space maintainer can be fixed (cemented) or removable. If a child prematurely loses a baby tooth, a space maintainer is used to hold that space until the permanent tooth is ready to erupt into that space.

Self explanatory: It's a dental device to preserve space if a baby tooth is lost prematurely. Can be fixed or removable. By preventing teeth from shifting the appliance it lessens severity of malocclusion, shortening or eliminating the need for braces. Either way, the cost of the appliance will be more than covered by the reduced cost of less complex Orthodontics.

To hold space: this is usually used when there is a concern of a loss of a given space in the mouth for teeth which could lead to much more complicated problems