This is how i understand it: electrical conductivity (EC) of your water can be measured with a calibrated electric circuit/probe (think voltmeter) and this value is mostly sensitive to certain disolved salts, most of which are used in nutrients/fertilizer products, so EC is a good measure of relative "strength" of your nutrient mix. EC can be converted to salinity and then also TDS based on assuming some conversion factor of percentage of salts that contribute to the measured EC value (nitrates for example do not contribute significantly to EC, but contribute eaqually to salinity/TDS). I think the sample can also be tested directly for TDS by separating said disolved solids +salts and weighing to get a percentage of total fluid sample weight.
