It is not uncommon to have problems with your pool, regarding the sand that makes the water dirty. The more you use the pool the more sand there gets, especially if you don't use special swimwear. If the pool is uncovered most of the time, it is advised to use a swimming pool sand filter which will remove all the sand that it can.
Normally an otherwise clean pool can look really dirty if the water has sand in it, and especially if the bottom of the pool is filled with sand. It can be difficult to remove manually since the water with the sand in it, can't be picked up with traditional pool cleaning equipment. A pool sand filter does the job for you.
In a sand filter like this, there wil be a vessel where the water is pumped. A pipe mechanism will lead the water on the top of a filter and then let the water rain at a natural rate through the filter. This way it is going to be filtered more efficiently - if it would be filtered with pressure, a part of the sand would get through and damage your pump and other water filters.
Under the filter there will be another vessel which is going to gather the filtered water and lead it forwards to possible other filtration or water ionizer, and then to the pump and back to the pool for another round. These sand removers usually clean themselves, but at occasion you might need to clean it manually, by removing it from your pool.