The air filter stops airborne contaminants from getting sucked into your car's engine. It's a paper filter that stops debris such as dust and leaves before they get inside the engine and do damage. If your air filter gets too dirty or clogged, your engine won't be able to suck enough air into the combustion chambers. The engine will then run rich (i.e., too much gas and not enough air). When this happens, your car will lose power and run roughly. Your Check Engine light also may come on.