Many older later model and especially newer model vehicles have cabin air filter that traps dust, pollen, and other airborne debris. It is a feature designed to make the air in the interior cleaner and can be particiular helpful for those with allergies or other respiratory problems.
Just as the filter for your vehicle's engine requires periodic replacement, so does the cabin air filter. Replacement intervals vary per manufacture, but I always recommend to check it often and replace as needed. If it has been more than 1 year since you last changed yours--you are probably due for another one.
If you are planning to tackle this project yourself, great! Just make sure you are installing a filter of equal into the system and make sure you follow the instructions of the direction in which you insert the new filter. Another good habit is to clean out the cabin air filter housing as it can become full of leaves and other debris.