A MERV rating refers to the scale on which filters are rated. This rating system was developed by ASHRAE and stands for minimum efficiency reporting value.
The higher the MERV rating, the more particles the filter can capture, and the smaller those particles can be. Particle sizes range from 0.3 and 10 microns (µm).
The MERV scale runs from 1 – 20.
For Example:
A MERV 1-4 filter can only capture particles 3.0 – 10.0 in microns. A MERV 14, common in critical areas like operating rooms, can capture particles as low as .30 microns.
HEPA filters can capture even smaller particles with a much higher efficiency rating.
References: https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating