Tin foil on steroids. Originally called Multi-Layer Insulation by NASA who invented it for the Apollo missions, it is typically two sheets of foil with a different type of thin insulation between them, either bubble wrap or fiberglass. They reflect radiant energy. Sunlight is the best example of radiant energy. Campfires are another example. Our bodies also produce a lot of radiant energy which is why emergency road kits often include a foil blanket. Beware, though: lots of snake oil surrounds this product.