The TDS (Total Dissolved Solids) meter measures the electrical conductivity of water. In its purest form, water is not a particularly good conductor of electricity. However, when other chemicals, many of which are conductors of electricity, get dissolved in water, its electrical conductivity goes up. Measuring the increased conductivity provides an estimate of the total dissolved solids in the water.
A good approach to using a TDS meter is to take a reading of the water from the kitchen tap, and then one from the water you are measuring. If the readings are within 10% of each other, then the test sample is reasonably free of dissolved solids in relation to the cleanest water available at the property. This is only applicable if the water is from normal mains, and does not apply to well / bore supplies.
