The cabin air filter is typically a pleated-paper filter that is placed in the outside-air intake for the vehicle’s passenger compartment. Some of these filters are rectangular and similar in shape to the engine air filter. Others are uniquely shaped to fit the available space of particular vehicles’ outside-air intakes.
Clogged or dirty cabin air filters can significantly reduce airflow from the cabin vents, as well as introduce allergens into the cabin air stream. Since the cabin air temperature depends upon the flow rate of the air passing through the heater core, the evaporator, or both, clogged filters can greatly reduce the effectiveness and performance of the vehicle’s air conditioning and heating systems.
Description from https://en.wikipedia.org/wiki/Air_filter