Also referred to as TDS (total dissolved solids) or EC (electrical conductivity). It's simply a measurement of any additives in your water. It doesn't decipher what type so it's just used to determine the strength of a solution.
When measuring the ppm, the main thing you're measuring is the NPK. The ppm of your water and additives like Hydroguard, amino/fulvic/humic acids, carbohydrates, vitamins, enzymes, etc don't contribute to nute burn.