Ken Climo signature discs say "XX-time champion" or something to that effect. 12x or any other number-x is the number of championships he had won when that disc was stamped, so people use it as an approximation of the run of discs.
Run refers to the batch. When discs are manufactured, they are run in enormous batches and first run means it was in the initial public release, second-run in the second batch made, etc.
There are usually minute, but sometimes gross differences between different runs of the same disc. People get used to one run and want the same thing again. Personally, I don't worry about it unless they changed the mold. Discs' flight characteristics change so much with use, wind, and weather that I generally find some difference in feel, but little difference in flight between runs if the mold hasn't been changed.
When life gives you lemons, stick them in the freezer for a few hours, then throw them back, 'cause they're like rocks, man.