Oh man this one can be an in depth answer, but Ill try and keep it short.
PH- acidic levels in the water (HYDRO likes the water to be around 5.6-6.2 while SOIL like to be in the 6.5 range) to test for ph just buy and digital meter or liquid drops from your local or online grow shop
PPM (PARTS PER MILLION) - this is the level or how much nutrient solution is in your water. (PPM will basically tell you how much nutes you put in)
If you are starting plants you would want a very low PPM (the higher the PPM the more nutrients you are giving your plants) as young plants cant handle high nutrient levels, as well as a few select strains do not need alot of nutes either.
Use the pyramid technique when feeding. Start with small amounts or nutes (or if you can buy a ppm meter and measure it that would be best) start at 200PPM and work up. As the plant gets older and stronger jump to 400PPM or increase nutes. During flower you would want to up your PHOSPHUROUS and POTASIUM (this will increase your PPM as well) anything you ADD to the water will increase PPM. Once youve reach anywhere from 1000PPM to 1800PPM mid flower, start backing off as you want to start FLUSHING OR CLEANSING the plant of the salts and chemical build up (unless organic) youve been pumping through them for the duration of the harvest