When the flush handle is pushed, its linkage lifts the stopper off the discharge-pipe opening and the tank empties into the bowl flushing it. The stopper floats back down on the dropping water level to plug the opening again while the float sinks down to open the valve. The weight of the water in the tank holds the stopper in place until the handle is tripped. Too much water in tank. Adjust the float arm by bending it until the level at each filling is correct.
Most tanks have a water level mark inside.
This mark is one inch below the top of the overflow pipe. Continuously running water. The stopper gets worn and soft or mushy and will not seat properly. A new stopper with the wires is easily installed. It must be positioned to drop straight down when released, and the wires must be straight for correct seating.