The transit areas are defined for travellers which are making en route stops while changing flights and so on. For some transit areas, you need a visa, for others you don’t.
In order to be eligible for this visa-free transit, you still need a booked international onward flight. You are not allowed to return to the country from which you just arrived from. After transit, you need to leave for a third country.
China offers three possibilities of entering the country without a visa depending on your passport: 24-hour TWOV, 72-hour TWOV, and 144-hour TWOV. After these time periods, your transit must end and you must depart China.