Almost every air conditioning system has a filter upstream of the evaporator coil. This can be in the return grille or in special slots in the duct system and can be a fuzzy-looking or a folded paper filter. This filter removes particles from the air stream to both keep the air conditioning system clean and to remove particles from the air.
As the filter does its job, it gets loaded with more and more particles. This actually has the effect of making it more efficient, but it also increases resistance and reducing airflow. When this happens, it is time to change the filter. How long it will take to happen depends on how dirty the air is and how big the filter is.
If you don’t change the filter, the air flow will go down, and the system will not perform well. Not only that, but if the filter is too dirty, it starts to become a source or air pollution itself.
If you take the filter out completely, you would solve the low air flow problem, but this victory would be short lived. The particles that the filter would have taken out will now build up on your evaporator coil and eventually cause it to fail. A new filter is a lot cheaper.
When you do buy a new filter, ASHRAE recommends getting one with a Minimum Efficiency Rating Value of MERV 6 or higher.