Fibreglass is a light, strong material made from very small glass threads pressed together, often set in plastic. People use it to make sports cars, small boats, surfboards, and helmets: it is lighter than metal and does not rust. Though actually, English stretches the word. The fluffy pink or yellow stuffing in a loft, sold to keep heat in, is also called fibreglass, and that stuffing itches. The hard boat-shell and the fluffy insulation are cousins, not twins. It is also a verb: to fibreglass a crack is to cover and seal it with this stuff. American English writes fiberglass. Same material, same uses.