PPM's means - Parts Per Million.
For example: Basically, your ppm reading (taken from a meter you dip into your nutrient solution. you will have to purchase one if you go hydro) tells you how many parts, out of a million, of that liquid is nutrients. A higher ppm reading means less water and more nutrients (stronger). Lower number, the opposite (weaker).
EC is another reading you will get familiar with if you go hydro. It is the Eliectronic Conductivity of the solution. Measured the same way, it tells you the concentration of salts in the liquid being measured. I don't really use the EC much, but I am paying attention to what it is when I check my nutrients (nutes) so I will be familiar.
If you don't have a meter, get one (if going hydro). Do not skimp on your meter. I mean, get a good meter, not just the basic model, you will be happier. I first bought a cheap PH meter to check my PH and then I bought a good meter that reads the PH, PPM's and EC. I wish I had just bought the good meter first. I use the cheap one to double check the good meters' PH reading.
Both meters are from Milwaukee (that is the company name, not where I got them). I had some issues with my original purchase, but they rectified the problem and went beyond their call of duty, sort of...
Also, before yo buy expensive stuff, be sure you can return the equipment if you need to. I bought a decent meter for $160.00 (I think that was the price) from a hydro shop in Canyon Country and it didn't work properly. The shop informed me AFTER the purchase that there are no returns or refunds. I guess there were signs posted in the store, but I didn't read them before I bought it. Needless to say, I had to pay for shipping a meter back to Milwaukee 3x's and pay for shipping myself, all the while (3 weeks, I think) my plants were not being given metered nutes... I had the cheap meter to keep PH in check at least.
Because of that, I told the hydro shop that I will never shop there again, and haven't bought anything that costs over 5 bucks from them since. To make it worse, the last time I was there to buy something cheap, the counter girl added something extra to my order and over charged me. I get home and see the item in my bag so I have to drive back to the store to return it.
They also gave a freind some pesticide to put in his grow (a bomb I think), that is not supposed to be used on plants, according to him. I've concluded taht the girls that run the place are fun to look at, but they don't seem to know shit about growing and I promise you, they do not know shit about customer service. -but they are cute girls and they will probably get by using their "talents". Haha!