A TDS (Total Dissolved Solids) meter is a device used to measure the total concentration of dissolved substances in water. It works by measuring the electrical conductivity of the water, which increases with the concentration of dissolved solids such as salts, minerals, metals, and other substances. TDS meters are commonly used in various applications such as testing the quality of drinking water, monitoring water purity in aquariums, hydroponics, pools and spas, and in industrial processes where water quality is critical. The measurement is typically expressed in parts per million (ppm) or milligrams per liter (mg/L).