A door threshold is a barrier at the bottom of a doorway that prevents water, insects, and debris from entering a building. It also helps ensure drainage, protect flooring, and provide accessibility for wheelchairs and mobility devices. Thresholds come in various materials and components, and their specific design and functionality vary depending on the type of door (exterior, interior, sliding, or French). Regulations such as the ADA and IBC set requirements for thresholds, and resources like door manufacturers, home improvement stores, and contractors can assist with selection and installation. Regular maintenance and inspection are crucial to ensure proper functionality and prevent damage.