new to aeroponics -- what is ppm? and how do you measure it?

Hey all!

I have some Mistys and Jack Herrer's in veg that I'm going to clone and place into stinkBuds aero system, and I've been reading a few of the aero threads before i get started, but i cant get ahold of this ppm thing.

ive googled it and i know it stands for parts per million, but is it like a "tick" in the cap of the nute bottle that you fill to? like 1/4 cap = 500ppm?

or 1/4 cup per gallon of water will give you 500ppm?

and if so, how do you measure it a day or so later?

ive googled ppm measurer and gotten nothing. is it like ph, with the strips?

stoned and clueless...

You buy a tds tester