The term, "Particulates" refers to particles of rust, dirt, sand, and sediment found in drinking water. Water that contains Particulates is typically cloudy, and as you might expect, tastes and smells "dirty". By themselves, Particulates are not known to cause any adverse health effects, but they can sometimes carry other harmful organic, inorganic, and microbiological contaminants through the drinking water system.
In general, better filters will remove finer particles. Each filter's ability to remove particulates is measured against a certain "particulate class", or particle size.
Class I - 0.5 to 1 micron
Class II - 1 to 5 microns
Class III - 5 to 15 microns
Where one micron is one millionth of a meter or approximately 1/25,000 of an inch.
For example, if a filter is rated to remove "95.1% of Particulates (Class II)", that just means that it will remove 95.1% of all particles that are between 1 and 5 microns in size