What N95 Mean in “N95 Respiratory Dust Masks”?

The first digit is a letter that corresponds to its resistance to oil.
The second and third digits are numbers that denote the percentage of particulates the respiratory dust mask can filter out. For example, a P95 respirator is an oil proof respirator that filters out 95% of particles, and N95 means not oil proof respirator masks that stops 99% particles that are at least 0.3 microns.
Letter:

1. N - Not Oil Resistant
   

2. R - Resistant To Oil

3. P - Oil Proof

Number:

1. 95 - Removes 95% Of All Particles That Are At Least 0.3 Microns In Diameter

2. 99 - Removes 99% Of Particles That Are At Least 0.3 Microns In Diamter

3. 100 - Removes 99.97% Of All Particles That Are 0.3microns In Diameter Or Larger

And in dust mask production industry, N95 dust mask also means the type or shape of the masks: the cone type/cup type masks, like 3M 8210, 8511, to  distinguish these masks from flat surgical masks.