All tank-type flush toilets operate the same way. You can observe the toilet's inner workings by removing the top of the tank. Set the lid in a safe place, laying it flat so that it doesn't fall and break. As the tank empties, the large ball, called a float, attached to the end of a long rod falls with the water level in the tank. At the other end of the float arm is the intake valve, which opens as the float moves down.
Water feeds into the tank as the intake valve opens.
When the tank is almost empty, the tankball falls into the outlet, stopping the flow of water to the bowl. The tank then begins to fill from the open inlet valve. A run-on toilet is usually caused by a malfunction in the tankball, the inlet valve, or the float. After you find the source of the problem, the next step is to get in there and fix it. Time for a new float.