Ok so lets say you have 1 million CC of water. ok and you take out 500 cc of water and replace it with salt. now you have 500 cc out of 1,000,000 cc or in other terms 500PPM (Parts Per Million) the higher the PPM the more salt in the water.
you can also measure using ec which is electrical conductivity. Pure water should have no conductivity so as salts(nutrients) are added they are conductors and can be measured in the water using a meter. My meter measures the EC and PPM.