What does micron size mean and how to select filter cartridges based on it?

The size of particles filtered for a given cartridge (mechanical/sediment) is called micron size. Filter cartridges are selected on the basis of user requirements and the degree of water pollution. 

For pre-filtration purposes cartridges with filtration parameter of for example 100 microns (µm) are selected. Following them are 50, 5 or 1 µm rated cartridges. 

The smaller micron size (number) the more accurate filtration occurs thus a greater pressure drop.