EC is electric conductivity in a liquid. Plain distilled water has an ec of 0 because there are no total dissolved solids (tds) in it. The more nutrients in the water the more tds - therefore the higher the ec reading. The meters that most of us use to measure ec is actually an instrument to measure ppm (parts per million). These meters convert ec into ppm to tell us what strength our nutrient solution is. Different manufacturers use different conversion methods to determine ppm. Some are 500 ppm = 1.0 ec and others are 700 ppm = 1.0 ec, so it's important to know what conversion method is used for what meter you have. For instance I have a Hanna ppm meter that has a conversion of 1.0 ec = 700 ppm. So when my ppm meter reads 800 ppm - that is equal to 1.14 EC. It's very important for your plants health to know how strong your nutrient solution is, otherwise you can fry it with too strong of a solution.