Your engine has four basic needs - air, fuel, compression, and a spark. The engine air filter is responsible for preventing dirt and debris from entering the engine through the air intake.
During the combustion process, each cylinder in your engine breathes in air on the intake stroke by way of vacuum. Fuel is added to the air in a precise measurement. The pistons compress the air/fuel mixture, and a spark ignites the compressed gases. Your engine goes through this combustion cycle thousands of times every minute that it is running. If dirt is able to get inside the cylinders, damage to internal engine parts - the valves, pistons, and cylinder walls, to name a few - can cause significant engine problems. Therefore, an air filter is added to the passageway through which the air travels to the engine. Over time, the air filter becomes clogged and needs to be replaced as part of routine vehicle maintenance.