AC Filter or Cabin Filter serves to filter the contaminants in the air entering the cabin through the AC ducts. They remove dust, pollen and other pollutants from the air you are breathing while driving along.
It is recommended to change the cabin filters every 12 months and it will ensure a fresh and clean environment while driving. Some of the indicators to look out for suggesting a cloggy filter is that you may experience reduced Air Circulation, musty odor, reduced air conditioning performance etc.