Underlay may refer to flooring or roofing materials, bed padding, or a musical notation.
Underlay or underlayment[1] generally refers to a layer of cushioning made of materials such as sponge rubber, foam, felt, crumb rubber, or recycled plastic; this material is laid beneath carpeting to provide comfort underfoot, to reduce wear on the carpet, and to provide insulation against sound, moisture, and heat.
Specialist underlays include ThermalStream which have air perforated holes to allow a faster heat transaction.
Underlay for timber floors is typically 3mm closed cell plastic foam.
Especially if there is spillage it will otherwise take a much longer time for the moisture to disperse and evaporate if it is prevented by the barrier from seeping through the wooden flooring.