MERV (Minimum Efficiency Reporting Value) is a 1-to-16 score for how well an air filter traps particles. A higher MERV catches smaller particles. The rating comes from an industry test (ASHRAE Standard 52.2) that measures how much a filter captures across particle sizes from 0.3 to 10 microns — a micron being about one-hundredth the width of a human hair.
For a buyer, MERV tells you what a filter will and will not catch. Low numbers (MERV 1 to 4, the cheap fiberglass kind) mainly protect the equipment by stopping large dust and lint. Mid-range filters (MERV 8 to 10) are the common household choice and catch finer dust and pollen. The U.S. EPA says that if you upgrade, choose at least MERV 13, which captures particles in the size range associated with smoke and common airborne irritants.
Higher is not automatically better, though. A denser filter is harder to pull air through, so it raises the pressure your blower fights against. On an older system with a basic fixed-speed motor, a too-restrictive filter can choke airflow, cut efficiency, and strain the blower. Many homes handle MERV 13 well — especially with a variable-speed ECM motor — but check your equipment's filter limit before jumping to the highest number.
MERV is also not the same as HEPA. A true HEPA filter meets a separate, stricter standard (capturing at least 99.97% of 0.3-micron particles) and usually needs its own dedicated equipment, not a slot in your existing furnace. For most homes, a MERV 11 to 13 filter that your system can handle is the practical balance between clean air and good airflow.