It is very similar in appearance to an engine air filter. Depending on where you live, the filter may get cloggedup with dust, bugs, seeds, leaves etc and eventually won't draw air into the cabin.
So you should check it as recommended and either change it, or if you have an air compressor, blow it clean. This appears to work OK on mine. The old rule of thumb was to take a paper filter and hold it up to the sunlight. If you can see light thru it, OK, if not then replace or blow clean.
Some replacement filters have a carbon pack which I assume eventually dies so blowing it clean won't rejuvenate the carbon.
Good Luck.
Jim Henderson