Ph is the measuement scale given to water and indicates how acidic or base the water is. What Ph you need depends on what type of medium you grow in. For hydro or rockwool mediums usually it's abt 5.5 or so, for coco users usually a little higher. For soiless growers (Pro-Mix or Sunshine) abt 6.0 and for soil abt 6.3. Keep in mind this may vary some depending on strains, nutes used, etc, as they say your mileage may vary.

PPM stands for parts per million. Using a meter designed for this, you can measure the amount of nutrients you have in your water or find how much crap is in the water you are starting with.

Hope this helps you.
OM