Dumb Yarn question regarding numbers

Can anyone explain to me what these numbers mean in reference to yarn? Since I'm not a knitter, I have no experience with commercial info about yarn, and I just can't seem to find any good references that explain this. The only yarns I have used are basic acrylics and now I have my own handspun, but I don't get the grading system at all...

So what is 20/2 or 10/2---I'm kind of guessing that the 2 may refer to ply, but the other number is escaping me entirely.

Please help!