A sealer is a substance that is applied to a surface to protect it from damage or deterioration. Sealers can be used on a variety of materials, such as wood, concrete, stone, and metal, and can serve different purposes, such as waterproofing, UV protection, and resistance to stains. There are different types of sealers available, including penetrating sealers, film-forming sealers, and reactive sealers. The type of sealer used will depend on the material being sealed and the desired level of protection.