Inside the toilet tank is a tall tube, usually it is white and stands above the toilet flap in the middle of the tank. The tube is open at the top and should stand taller than the surrounding water level. This tube is referred to as the overflow tube or drain tube. Water that goes down this tube goes directly to the toilet bowl.
The purpose of the overflow drain is two-fold. It's primary purpose is to channel water directly into the bowl for purposes of refilling the bowl with water after a flush. It's secondary purpose is to prevent flooding by channeling any tank overflow water into the bowl and ultimately down the drain.
If You Hear Continuous Running Water
The water level in the tank, when it has finished refilling, should be below the top of the overflow tube. If the water line is at the level of the opening, water will flow out of the tank and down the drain. If the water level is set too high, the fill valve will continuously let water into the tank while the excess water will continuously flow down the drain. This is the common cause of hearing the continuous sound of running water. If the water level is too high, reset the water level by adjusting the float so that it shuts off the fill valve sooner.
If The Bowl Doesn't Refill With Enough Water
There is a small tube that leads from the fill valve to the overflow tube. It should be fastened to the overflow tube so that water is directed down the overflow tube when the fill valve is running. The fill tube refills the bowl with water after a flush. If the bowl doesn't not have enough water after a flush cycle completes, check that the fill tube is connected and directing water only into the overflow tube.