MPR is an acronym which stands for Micro-particle Performance Rating and is the system used by 3M, the manufacturer of Filtrete filters, to rate their filters. The scale begins at 300 which is the lowest rating and goes up to 2200 which is the highest rating on the scale. Like many manufacturers, they utilize electrostatic characteristics to enhance the ability of their filters to capture dust particles.
Rather than using the industry standard MERV rating system which measures .1 to 10 micron particles, they created their own rating system which measures .3 to 1 micron sized particles. One system is probably no better than the other, they are just different. The higher the MPR rating is the more efficient the filter will be at removing dust particles from the air.