The United States National Institute for Occupational Safety and Health certifies filters as N95 when they can block 95% of airborne particles larger than .3 microns across. N95 respirators fall into the category of "mechanical filters," which protect breathing passages from particulate mater like dust, bacteria, and some—though not all—viruses. They won't, however, protect against the infiltration of oils, vapors, or gases.
The "N," in fact, is a mask rating that stands for "non-resistant to oil." Other ratings include "P" for "oil proof" and "R" for "oil resistant." The polyprophylene fiber from which they are constructed carries an electrostatic charge, which can be damaged by alcohol or soap, so N95 masks are difficult to clean without degrading their effectiveness.
The U.S. Food and Drug administration recommends N95 masks and filters for medical professionals, since they can help obstruct the droplets that carry communicable illnesses like COVID-19.