it measure the electoconductivity in your water ( you prolly thinking whats that) in simple terms for a grower when you are making a feed for your plants (water + nutrients) it measure how much nutrients you have put in the water (measure the plain water with your ppm, then put your nutrients in the water and measure the ppm it will rise) if you follow a nutrient guide it will tell you what ppm you should be feeding your plant at different stages of your grow..... hope that helps