The average sand filter has a tank which contains filter sand & sometimes a bed of gravel.
The valve assembly can be mounted o top or on the side or across the base with older style units.
The Valve commects to the pump outlet & the water goes down through the sand in the filter cycle.
The rubbish is collected in the sand & the clean water passes through the base laterals & back up the center pipe to the valve & back to the pool.