The overflow tank (sometimes called the expansion tank) is the reservoir where excess engine coolant is stored during normal operation. As the coolant heats up, it expands, and enters the overflow tank. Once the driving session is over and the car cools down, it retracts, creating a vacuum and pulling the overflowed coolant back into the radiator.

Because these are plastic and mounted low, it's fairly common for them to crack and leak. A leak in the overflow tank will not typically cause you to lose all of your coolant at once, but gradually lose coolant over time, as only excess coolant sent to the overflow would be given the chance to leak.