TDS = Total Dissolved Solids, which is any salt, metal or mineral in the water. With the push of a button, the TDS-EZ can tell you your overall water purity level, which will be displayed on the screen in ppm (parts per million). The lower the TDS level, the purer the water, with 0 ppm being pure H2O.
https://www.amazon.com/HM-Digital-TDS-EZ-Measurement-Resolution/dp/B002C0A7ZY

