It tells you how many "Total Dissolved Solids" aka, TDS aka EC (Electrical Conductivity). As others have posted, a PPM meter tells you exactly how many solids out of 1 million are dissolved into your liquid (water). It doesn't tell you exactly what nutes or how much of each type of nutrient are in the water. It just tells you that out of the 1 million particles of water, there is 1000, or 1500 solid particles or whatever the reading is. What those particles actually are isn't specifically known.