Ok, I'm no pro but been at this a while. I'll break it down like i know it. Ppm is basically the amount of nutrients/minerals in your water. Ppm means parts per million. But don't trip in the technical shit. It gets intimidating. Ro water is 0ppm and tap can range from 25ppm to 400ppm is what I've seen depending on where your at. When you add nutes to water you're raising the ppm. It's just a accurate way to measure the ammout of nutrientsi n the water. And to see if the plants are using up nutrients after they have drank the water down.
Basically I use a pen on the 500 scale. More technical shit, but you need to know which one to buy. if you're gonna use what I say.
Depending on nutrients 150-250 ppm is in the 1/8 to 1/4 strength range. And the nutes I've used in hydro 1000ppm is usually the max I ever run. And full bottle strength is usually close to that.