Total dissolved solids

TDS counts salts, minerals, chemicals, and other matter dissolved in your pool. It rises over time because water evaporates but the dissolved material stays behind.

High TDS can make water look dull, cause scale, and make chemicals act oddly. It also reads high in a saltwater pool because salt counts toward it, so compare to your own baseline rather than a general number. The only real fix for high TDS is a partial drain and refill. Your fill water starts with its own TDS, so test it when you add water.