The purpose of the air filter is to

The purpose of the air filter is to remove dust particles, chemicals or biological hazards from the air. Air filters are used extensively in vehicles, personal protection, gas turbines, laboratories, pneumatics and air conditioning systems in buildings such as homes and hospitals, as well as in indoor air purification. The filter element of the air filter may be, for example, activated carbon, paper, fiberglass, cotton, foam or the method may be electric.

filter out your air some things like dust and dirt/. The purpose of a car's air filter is to prevent dirt and other particles from entering the fuel system or engine. Air filters in a house also prevent particles from entering the air conditioner system.