TOTAL DISSOLVED SOLIDS
Total dissolved solids (TDS) is, as its name implies, the measure of the total amount of dissolved material in the water.
The level of TDS in the pool is influenced by many factors; the chemicals we add to adjust the pH, chlorine, water hardness, alkalinity, dust, dirt, human waste, . . . all increase its level.
The maximum acceptable level of TDS for swimming pools is 1,500 ppm. At values above this, we begin to notice stains in the pool. It will also reduce the activity of any chemicals you add, preventing them from doing what they're supposed to. The water may also become cloudy.
Unfortunately, if the TDS level is too high, there is little that can be done to reduce it. The only practical solution is to drain some or all of the water from the pool (depending on how high the TDS is), and replace it with fresh water (with a low TDS). A regular backwashing routine will help to prevent the TDS from rising too much.