As viruses go, the coronavirus is a very small virus. This is a great picture of and comparison of various viruses by size. An N95 mask filters 95% of 0.3 micron particles. So why is it helpful for a virus that is only 0.1 microns?
The short answer is that in reality, you don’t really aerosolize out a single virus. Coughing or sneezing or even just talking creates aerosolized particles in the surrounding air. Most of these particles are 1-20 microns in size. But there are sub micron particles as well.
If you want to go full on nerd about filtration, I highly recommend this read by 3M which is highly helpful on the subject of filtration in regards to healthcare. There are several different mechanisms of filtration. “These are gravitational settling, inertial, impaction, interception, diffusion, and electrostatic attraction.” The combination of these mechanisms results in an actual filtration that is actually higher than 95%.
It is important to note that the weakest link in your N95 is the seal. You can have a great filter but if it does not fit properly, it will not filter the air you breathe.
This is also a good over arching read of the basics about respirators here.