A fill valve is a crucial component in a toilet's flushing system. It controls the flow of water into the toilet tank after a flush, ensuring the tank refills to the proper level. When the tank is empty, the fill valve opens, allowing water from the water supply line to flow in until the desired level is reached, at which point it closes automatically. There are different types of fill valves, including ballcock and floatless designs. Each type operates slightly differently but serves the same purpose of maintaining the water level in the tank. Proper functioning of the fill valve is essential for efficient toilet operation and water conservation.