A moisture barrier is a layer intended to limit moisture passage at a particular position within a building element. Its suitability depends on the moisture source, substrate and junctions; the name alone does not describe a complete protective system.
How this term relates to your project
When a floor is replaced, moisture from below may affect the proposed finish. Ask which source the barrier is intended to address and what assessment supports that choice. A barrier beneath a floor has a different position and purpose from a vapour control layer within a roof. Request details showing the junctions with walls, penetrations and existing floors.
These locations can introduce interruptions or damage that are difficult to inspect later. Compatibility with the screed and final floor covering also matters: identifying a product does not establish that every combination will work. Keep the specification and installation photographs in the project records before the layer is concealed.