Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.
Summary, TDS gives you a reference value what is dissolved in your water in the form of waste, minerals, liquid fertilizers, etc. Another crucial maintenance tool beside TDS meter especially for water monitoring. Most of the hobbyist will have at least one unit.