Air exists in space. Anyplace there is space and gaseous molecules are introduced, there it subsides. Air also subsides in some liquids, like carbonated beverages. It occasionally lives in porous solids like brick and bread.
It usually means that the air conditioner has low refrigerant. You need to get the compressor recharged - but beware - if the unit IS low on refrigerant, then you might have a leak in the cooling...