This table represents the 3 ILS Categories and their required Minimums*
*Minimums = if the weather condt are lower than those described in this picture > Approach cannot be flown!
You approach a given airport with a weather report (METAR) telling you that the RVR is below 2.400FT (VIS < 1.800ft). In that case it would be a Category 2 ILS Approach. However the airport you are approaching must have or offer the CAT2 Approach (Approach Plate) as well. If not the case, you can not fly this approach.
Again, as with any other approach, its weather minimums have to be met in order to be allowed to fly it!