Pools with a sand filter will have a multiport valve after the pump to change the direction of the flow of water depending on what you're trying to accomplish. This article will help you understand what each mode does.
Filter: This is your normal operating mode. Water goes into the filter and back out to the pool.
Recirculate: In this position the water bypasses the filter and just recirculates right back into the pool. You'd be in this mode when trying to prime your pump or if something on the filter was leaking but you still wanted to circulate the water.
Closed: This stops water at the valve and is really only used if your equipment is below water level and you need to work on the pump. NEVER turn a pump on when the hand is in the closed position. The water will have no where to go and can ruin your system.
Backwash: This reverses the flow of water inside the filter to clean the sand. Water goes in the bottom of the filter, pushes the sand up and wastes the top layer of water out the waste line.
**Always make sure your waste line (backwash hose) is layed out and clear to have water flow out of it in backwash, rinse, and waste positions**
Rinse: After backwashing you will rinse the sand. Water goes in the top of the filter and back out the waste line.
Waste: If you need to drain water from the pool this is how you can do it. Water will go in the multiport and right back out the waste line
Winterize: This isn't a specific position on the valve but its best to have the spring slightly compressed and off the gasket inside during the winter.