Most people find out their car has a cabin air filter the day somebody points at it. It sits between the outside air and your vents, and it catches the pollen, dust, and road grit that would otherwise end up in your lap at every stoplight. It has nothing to do with the engine. It's the air you and your kids breathe on the way to work and school.
Florida is hard on it. Oak and pine pollen coat everything here for weeks at a stretch, and our humidity means whatever the filter catches stays damp instead of drying out. That combination loads a cabin filter faster than a dry climate would. If anybody in the car deals with allergies, this is one of the least expensive things in the vehicle that changes how a drive actually feels.
What you'll notice first. Air that used to blast now feels like it's pushing through a pillow. Fan speed three does what speed one used to do. Sometimes there's a smell the moment you first turn the AC on.
One honest boundary. A musty smell isn't always the filter. It can also come from the evaporator case behind the dash, which is a different job with a different fix, and our heating and AC repair page covers that side of it. We check the filter first because it's quick and inexpensive, and we'll tell you straight if the smell is coming from somewhere else.
On most vehicles it lives behind the glovebox, so checking it is a couple of minutes with the glovebox dropped down.