Thresholds are horizontal door sills that are placed under door frames to help insulate a building. They are made of various materials, such as wood, plastic, or metal, and have the shape of a long, flat strip. They are used to prevent cold air or water from coming in under the door and to improve insulation. They can also be used to prevent drafts or noise from outside from coming in. Thresholds are easy to install and are an effective way to increase comfort levels in a building.