A toilet tank is the upper part of a toilet that holds water used for flushing. When you press the flush handle, a valve opens, allowing water to flow from the tank into the toilet bowl. This action helps remove waste and refill the bowl with clean water. Toilet tanks typically have a fill valve that refills the tank after each flush and a float mechanism that controls the water level. They can vary in size and design, but most are made of ceramic or plastic. Proper maintenance of the tank is essential for efficient toilet operation and water conservation.