If you look into a toilet tank, the flush valve circular area where the water is distributed into the tank. Its function is to release flush water from the tank and into the bowl. The water flushes waste down the drain and refills to maintain the vacuum of the toilet drain. The flush valve is regulated by a flapper, which creates a water tight seal so that the toilet can hold water until it is flushed.
The water supply comes from the wall and screws into the tank, where it is regulated by a trip lever. The tank ball or floating weight triggers the trip lever and cuts the water feed when the water reaches the fill line. This fill line correspond with the volume of the tank. American toilets hold a maximum of 3.5 gallons of water, which is dispelled with each flush. The flush valve uses a flapper, which is controlled by the flush handle and held open by the floating weight.
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When the toilet is flushed, the handle lifts a chain, which opens the flush valve. Water drains through the valve and into the bowl. The flapper remains open until the tank completely drains of water and then closes on its own. The floating weight or tank ball will drop and wait for the water level to rebuild. When the weight reaches its prescribed height, the trip level cuts the water supply and the toilet can be flushed again.