TDS stands for total dissolved solids, and represents the total concentration of dissolved substances in water.

TDS stands for total dissolved solids, and represents the total concentration of dissolved 
substances in water. TDS is made up of inorganic salts, as well as a small amount of organic 
matter. Common inorganic salts that can be found in water include calcium, magnesium, 
potassium and sodium, which are all cations, and carbonates, nitrates, bicarbonates, chlorides 
and sulfates, which are all anions. Cations are positively charged ions and anions are negatively 
charged ions.  - A document from Safewater.org