Knotty, a micron is a unit of measurement. It's short for micrometer, which is a millionth of a meter. So, as was said it's to do with the size of the abrasive particles in the suspension. You're looking there at a compound of half-micrometer and of three and a half micrometer abrasive particles.
The grit ratings that go up as the abrasive gets finer has to do, I think, with the density of abrasive particles, which would go up as the particles get finer and closer together on a medium.
Hope that helps a bit. I'm also selling my Edge Pro Apex tonight. Just a heads up.