Coolant reservoir 

It’s also called as radiator overflow tank. It collects the expanding coolant that’s heated by the engine and and resend it to the cooling system when it looses enough heat.

As the engine coolant heats up, it expands and build pressure. When the pressure exceeds a optimum level, the excess coolant is transferred into the overflow tank.

As the engine coolant cools down, it contracts and pressure reduces. When it cools and contracts enough, the pressure in the cooling system is below atmospheric pressure, so it creates a vacuum. So the coolant travel back into radiator from overflow tank because of pressure difference.