A toilet tank is the upper part of a toilet that holds water used for flushing. It is connected to the bowl and fills with water after each flush. The tank contains essential components like the flapper, which controls the release of water, and the fill valve, which refills the tank after flushing. When you press the toilet handle, the flapper opens, allowing water to flow from the tank into the bowl, creating a flush. Afterward, the fill valve refills the tank with water until it reaches a certain level, ready for the next use.