MERV stands for minimum efficiency reporting value. It is a measurement used to rate how efficient an air filter is on a one to 20 scale, with 20 being the most efficient. The higher the rating, the higher the ability to trap small particles in the air.
Whenever you use your heating and cooling system, it filters the air in your home at the intake. The air then goes to the system's furnace for conditioning. It must pass through the filter before the air is warmed or cooled.
High vs. Low MERV Ratings
Basically, the higher the MERV rating, the smaller the pore's filter will be. The smaller pores allow less air to pass through at once. This way, the filter purifies the air from the particles that would otherwise circulate back into your home.
When the air filter has larger pores, then more air can pass through the filter at one time. This allows for the air to circulate back with greater speed, though some particles will escape back into the home. The average home has a filter with a MERV rating of 6 to 8. Places that have filters with a rating of 20 are usually highly sterilized environments like hospitals and science labs.
The filter that you use should reflect you and your family's personal needs. Someone with small children might benefit from a 10 filter, while allergy sufferers, elderly people, and pet owners might require as high as a level 16. Keep in mind that the higher the MERV rating, the more energy your air conditioning system must use to force the air through the filter. This is why you should only use a higher rated filter is necessary.
Personal preferences aside, the specifics of your existing air conditioning system will most determine what air filter you can use. When in doubt, consult an HVAC professional to find a good air filter match for your home.