Underlayment is a material placed beneath the surface layer of floors or roofs to provide additional protection, support, and insulation. For floors, underlayment is typically used to smooth out imperfections, reduce noise, and provide a moisture barrier, especially in areas with high humidity. On roofs, underlayment acts as a secondary layer of protection against water infiltration, helping to prevent leaks in case the outer roofing material is damaged. It is commonly made of materials like felt, foam, or synthetic fibers, depending on its intended use and location.