A vapor barrier is a material used to prevent moisture from passing through walls, floors, and ceilings. It is typically made of plastic or foil and is installed in areas where humidity is high, such as basements and bathrooms. By blocking moisture, vapor barriers help protect structures from mold, mildew, and water damage. These barriers are often placed on the warm side of insulation in buildings to keep moisture from condensing within walls. Proper installation is crucial for effectiveness, as gaps or tears can allow moisture to seep through, undermining the barrier's purpose.