If you mean something like "1 Part A, 3 Parts B" it's just a simple ratio of 1 part of substance A, for every 3 parts of substance B. It doesn't matter how much you use, just like 3 gallons of water to 1 gallon of anything, or 300 gallons of water to 100 gallons of anything, it still maintains the same 1:3 ratio.
If you mean PPM, Parts Per Million, then I believe that stands for how many particles are in any given 1 million particles of the "atmosphere." And I use atmosphere in a loose sense because I believe PPM can be used in measurements of water, air, and any other chemical compound in which you could mix something. Not too sure about the specifics on PPM, but the general idea is how many particles in a given 1 million particles. Hope this helps.