Dew point is simply the temperature to which air must be cooled for the water vapor within to condense into dew or frost. At any temperature, there is a maximum amount of water vapor that the air can hold. This maximum amount is called the water vapor saturation pressure. If more water vapor is added beyond this point, it will result in condensation.
Dew point, simply, is related to non-pressurized, atmospheric air (atmospheric dew point). The lower the temperature of the dew point, the less amount of water vapour in the air.
When compressed air comes in direct contact with the product being produced, such as spray painting, packaging in food & beverage industries, semiconductor, air separation etc, with a high Dew point harms the product quality.
(See also, Atmospheric Dew Point and Pressure Dew Point)