The air we breathe is full of tiny particles of; chemicals, soil, smoke, dust or allergens, in the form of liquids, gases or solids.
These minuscule airborne hazards are referred to as particulate matter, or PM.
The 2.5 in PM 2.5 refers to the size of the pollutant in micrometers.
Here’s a diagram from the Environmental Protection Agency to help you visualize the scale of these tiny particles.