First, the designation 10/2 is the "lea" number. The /2 means it’s two strands plied. The 10 is a bit harder.
A lea is 300 yards, traditionally 120 wraps on a 90" skein. (Hey mom, remember how big my swift is? Now we know.) It weighs a pound. If your actual yarn weighs 5 pounds as a single, wrapped 120 times around my swift, then we have a 5 lea yarn. If it’s a double-ply, and weighs 5 pounds wrapped 120 times around my swift, then we have a 10/2 lea yarn. If it’s a quad-ply (is that a "cable"), and weighs 5 pounds wrapped 120 times around my swift, then we have a 20/4 lea yarn.
Does this sound right, Ann?
Even better, I read that "lea" is a 14th century word of measurement. This period just gets more interesting the more I get into it.
And just to keep things complicated, a 10/2 cotton is not the same size as a 10/2 linen or 10/2 wool. There’s a good comparison chart at http://www.knitting-and.com/spinning/ycount.htm