Hey man, I am new to growing myself and and in my first grow now, I did a quite a bit of research first and had your very same question.
PPM can be looked at as measuring the amount in your water that is NOT h20, therefore you take your water, tap, RO (reverse osmosis) or distilled, and you measure the pH, and PPM. These two numbers are important for this reason, pH lock out nutrients uptake of your MJ plants when they dip below 6.3 and rise above 6.8 in soil growth, different things get 'locked' out at different pH levels so it is imperative you keep it around 6.5 or in between 6.3-6.8. My natural pH from tap is a staggering 9.8, which would straight kill my MJ plants if I didn't know that and used it freely with watering without adjusting the pH down, most people in the country have tap between 6 and 7.5, which would be less severe but still a nagging deterrent to healthy growing MJ, but it would grow, though not anywhere near ideally and can also show symptoms of nutrient deficiencies even though you are giving it plenty of nutes, due to the lockout from high/low pH.
Another important thing, measure the pH of the water going in the when you water, and then measure the water runoff coming out of the pots, this can help diagnose the general pH of your soil. If the pH of your water going in is 6.8 and the runoff is 6.0 your soil is slightly acidic, dragging down the pH of the water you poured into the plants. If the soil-runoff measures 7.3, the soil is slightly alkaline, using pH medium managers like dolomite sweet lime (finely ground), in with your soil can help stabilize the pH so it flucuates less from the point of watering, to the soil run off pH levels.
On to your PPM question:
PPM below 300 is critically important, anything above 300 ppm usually indicate well water, or extremely treated city water. Most likely your PPM will be between 100-200 though, mine is 245 and I have city water, this is still acceptable but not ideal. More important though is the use of PPM as a measurement tool when giving nutrients, say you measure your PPM and it is 200, well there is nutrient guides that stipulate how much PPM you should give your plants in nutrient solution. So say your nutrient schedule (available online at most any nutrient manufacturers website) stipulates a PPM of nutrients solution of 150, you would want to put in your nutes, measure PPM and say it shows up 325 ppm, deduct your initial PPM of 200, and your PPM of nutrient solution is roughly 125 ppm, add slightly more nutes until the PPM measures out to 350.
Does that make sense? PPM meters will not delineate what the PPM in the water is, but just how much of the contents of your bucket is not water itself.
* A side note, you can call your local water provider, whoever sends your bill, and ask them if they use chlorine, or chloramine to treat your water. If you are using tap, if they use chlorine let your water sit out for a day or two and a chemical reaction with the air will cause the volatile chlorine to turn into a gas and rise out of your water and into the air, you can speed this up by using an aquarium bubbler that will force oxygen into the bottom of the bucket of water and up out of the bucket within a half hour. If you have chloramine as I do, you can use a product sold at Petsmart/Pet co, for aquarium enthusiasts that breaks the much more chemically-stable chloramine down into less stable components, a bottle which will last me for maybe a year was $6 dollars on sale at Pet Co. Both chlorine and chloramine are bad for MJ, assume if you live in a medium-large city one or the other will be present.
Copy-paste from wiki regarding chloramine:
Aquarium owners must remove the chloramine from their tap water because it is toxic to fish. Aging the water for a few days removes chlorine but not the more stable chloramine, which can be neutralised using products available at pet stores.
Many animals are sensitive to chloramine and it must be removed from water given to many animals in zoos.
Chloramine must also be removed from the water prior to use in kidney dialysis machines, as it would come in contact with the bloodstream across a permeable membrane. However, since chloramine is neutralized by the digestive process, kidney dialysis patients can still safely drink chloramine-treated water.
Home brewers use reducing agents such as sodium metabisulfite or potassium metabisulfite to remove chloramine from brewing liquor as it, unlike chlorine, cannot be removed by boiling (A.J. DeLange). Residual sodium can cause off flavors in beer (See Brewing, Michael Lewis) so potassium metabisulfite is preferred.
Chloramine can be removed from bathwater and birthing tubs by adding 1000 mg of vitamin C per 75 gallons of water. Human-grade nutraceutical vitamin C reduces the pH of the water less than commercial tablets sold for this specific purpose.
