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For most modern toilets there are two main components: an inlet valve and an outlet (or flush) valve. You can easily open the lid to your cistern (ie the outer casing) to see how they work in practice. When you push the flush button the outlet valve lifts up a rubber seal at the bottom (called a seating washer). This causes water to run out from the cistern into the bowl of your toilet pan to ‘flush’ the waste down into the sewers. When the water level drops it lowers a float on the inlet valve. The inlet valve will then open to bring new water into the tank until the level of the water raises the float enough to shut off the valve.