An air filter is an air admission element of an engine's air system that was created to filter air impurities that go inside the engine; this air is used in a combustion mixture. It´s principal function is to filter impurities present in the atmosphere, like pollen, soot and dirt particles, that can eventually bring problems in the engine.
The contaminant retention happens through a device (filter) which uses a pore media to remove or to separate impurities through a mechanical or physical process through filter elements like polyester, felt, fiber, paper, etc.
The air filter is the most important component to protect the engine and to avoid wear and this piece is important to control the pollutant emissions. If a car runs without a filter the durability of the engine will be affected.