Filter socks are listed by microns, but what exactly is a micron. Microns are a unit of measurement just like inches or feet. So just like inches or feet the smaller the micron means the smaller the opening to allow a particulate through. Basically, higher numbers mean less filtering and smaller numbers mean more.
The most common micron socks are 400, 300, 200 and 100. 300 micron will allow more through versus a 100 micron due to the openings in the material being larger. This also means it will clog slower, than the 200 micron or 100 micron counterpart. 100 microns are pretty small and usually clog within a day or two. They are however a great way to polish your water a bit before company is over. Remember, socks should be rinsed out every 2-3 days so you shouldn't pick a higher micron just so that you can leave the sock in for a week without it overflowing.