An underlay is a 1M by 50M thin, cushioning sheet of material—like foam, rubber, or felt—installed between a raw subfloor and your top flooring (such as laminate, wood, or carpet). Its main function is to create a smooth, stable foundation by leveling out small bumps in the floor and protecting the joints from cracking under weight. Additionally, it acts as a shock absorber that dampens noisy footsteps, blocks cold air from coming up, and stops moisture from rising and damaging your floors. Essentially, it serves as an invisible shield that makes your floors quieter, warmer, and much longer-lasting.