Some swimming pools use sand filters to clean and remove debris from the pool water. The sand in those filters gets dirty over time and needs to be cleaned periodically. A sand filter is backwashed in order to be cleaned. Backwashing pushes pool water back into the filter and stirs up the sand. All of the debris and other contaminates in the pool water get pushed up into the backwash line and sent out of the pool through the waste line.
Turn your pool equipment off. Lay the backwash line out on the ground where you want the dirty water to drain. Press down on the dial valve handle on top of the filter and rotate it to the "Waste" setting.
Turn your pool equipment on and let the pool run for at least five minutes or until the water coming out of the backwash hose turns clear. If using a plumbed-in waste line, watch the water through the sight glass near the top of the filter to determine when to stop backwashing.
Turn off the pool equipment again. Rotate the dial valve on the filter to the "Rinse" setting. Turn the pool equipment on and let it run for at least 30 seconds. Rinsing pushes the sand back down into the filter and prevents any from leaking back into the pool water. Turn the pool equipment off and rotate the dial valve to its normal setting before running the pool again.