Underlay is a thin layer of material installed between the subfloor and the finished flooring to provide cushioning, sound insulation, and moisture protection. It can be made from foam, rubber, felt, or fiber and often features an outer layer like foil or plastic film to act as a vapor barrier. Underlay improves comfort underfoot, reduces noise, extends the life of the floor, and helps smooth out minor subfloor imperfections. The type of underlay used depends on the flooring material, room conditions, and specific performance needs like thermal insulation or soundproofing.