The back of a toilet consists of a tank that holds the water used when flushing the toilet. Water drains from the tank into the bowl when you flush the toilet, pushing the bowl water and contents down the drain. If the tank water level is too low or you need extra water to add pressure to push down bowl contents that aren't flushing easily, you can add water yourself. This is not a permanent fix for a toilet that improperly flushes or constantly runs.
Remove the toilet tank top cover. Note the water level in the tank.
Flush the toilet while looking at the inside of the toilet tank. Make a note of where the water level stops. Look at the inside wall of the toilet tank for a water mark or a discoloration line.
Locate the water overflow tube in the tank -- a small tube near the top of the tank that siphons off water when the level gets too high.
Slowly pour water into the toilet tank until it reaches the water level discoloration or until it is about an inch below the overflow tube opening.
Flush the toilet to clear the toilet bowl and inspect the water level rising in the tank. Add more water as needed.