How’s that for a fancy title? Simply put, they’re better air filters. To understand the definition of “better”, you need to understand how they are rated. Air filters are given a MERV rating, or Minimum Efficiency Reporting Value. The numbers range from 1 to 20. The higher the number, the better the air filters are at removing airborne particulates of various sizes. A standard flat, 1-inch thick air filters have an MERV rating somewhere between 1 and 4. We call these rock catchers since about all they do is keep bigger airborne particles from getting inside your system and settling on motors, coils and other sensitive parts. The holes are too big to do much for smaller particles.
High Efficiency Media Filters are generally about 5 inches thick, are pleated to provide more surface area, and are installed in a rack designed specifically to hold these thicker filters. They do a much better job of capturing both smaller and larger airborne particulates and have a MERV rating of somewhere between 5 and 13. These filters are especially good for people who are allergic to dust, dander, mold, etc.
As a matter of fact, many older systems are not strong enough to pull air through 1-inch thick pleated filters, only the regular flat filters. Only a licensed HVAC contractor can take the pressure measurements needed to determine if your system can handle it. If your system can’t get the air it needs, all kinds of bad things will happen starting with louder than usual noises and an eventual freezing and catastrophic failure of the motor. And you really don’t want that.