The dew point is a highly accurate measure of the amount of moisture in the air. Unlike relative humidity, it remains constant and does not change with a change in temperature.
Technically, the dew point (which is reported as a temperature) is the temperature to which the air must be cooled for it to be saturated. In other words, if the air temperature is 70-degrees, and the dew point is 68-degrees, the air temperature would only need to be lowered 2 more degrees for the air to be saturated. Once the dew point and air temperature become the same, grab the umbrella! The air is saturated---and ready to unload its moisture.