Final answer:
Flashing is the waterproof material used over surfaces to prevent water penetration into buildings, particularly at joins or gaps. It is distinct from other materials like sealers and is necessary for proper building construction.
Step-by-step explanation:
Flashing is a waterproof material fitted over a surface where water is expected to run. The primary purpose of flashing is to prevent water penetration into a building and to direct the flow of water away from the structure. While options like sealer and waterproofing membrane also help in preventing water ingress, flashing is specifically designed to work at intersections of different building surfaces or at roof penetrations such as around chimneys, vents, and windows. It is usually made of materials like metal, plastic, or composite substances that have water-resisting properties.