If your car’s written off or stolen, GAP insurance (aka Guaranteed Asset Protection insurance) bridges the ‘gap’ between your car’s market value and the price you paid for it.
Car insurance only covers the market value, which could be less than what you paid due to depreciation, so GAP insurance makes sure you’re not out of pocket.
If you bought your car on finance, GAP insurance covers the price of the car when you bought it or your outstanding finance, whichever is higher.