A micron is one millionth of a meter (0.00003937 inches). By contrast, a human hair is about 100 microns in diameter.
It's important to understand what kind of micron rating you're using when selecting a filter bag. For historical reasons, most filter material is assigned a nominal micron rating based on 50% to 80% efficiency. Filters that remove a nominal 50% to 80% of all particles larger than 50 microns in a single pass are rated 50 microns nominal.
Filter bags made out of monofilament mesh, and multilayer microfiber are given an absolute rating. A filter bag with an absolute rating will capture 98%+ of its rated particles in a single pass. An absolute micron rating is generally displayed as the micron size, followed by A, as shown in the lower portion of the adjacent table.
Filters are sometimes measured in US wire mesh instead of microns. Wire mesh measures the number of wires (or threads) per linear inch, not the size of the holes between them. As the number of wires per inch goes up, the size of the holes (in microns) goes down. There is no simple formula to convert between microns and wire mesh because it would have to account for changing wire diameter.