What is an overflow tank in a vehicle for?

An overflow tank in a vehicle, usually part of the cooling system, is there to catch excess coolant that expands when the engine heats up. As the engine and coolant get hot, the coolant expands in volume. The overflow tank provides a place for this extra coolant to go, preventing pressure buildup that could damage the radiator or other parts of the cooling system. When the engine cools down, the coolant contracts, and it's drawn back into the radiator from the overflow tank.