Sand Filter (the large round thing)
This filter receives water from the swimming pool that has gone through the basket filters in the skimmers and past the chlorine tablets that have placed there (if it’s a chlorine treated pool) the water goes into the filter when the large black valve is positioned in the “filter section” this is marked on the handle surround.
Inside the filter there are up to 4 different grades of sand and gravel. These block even small particles and prevent them being passed back into your swimming pool via the jets.
If your filter is not working properly you may have no sand in the filter ( seen this more an once) your filter may just need a good backwash or there could be a problem with algae building up inside the filter over winter. There is a special sand/gravel cleaning product that you can buy to treat this.
Do not close the water inlet to the filter when you pump is on as the build up in pressure may fracture your filter ( seen this more an once and it is expensive !)