Dew point is the temperature at which the amount of water vapour which is present in atmospheric air becomes just sufficient to saturate it. At this temperature. present vapour pressure is equal o the saturated vapour pressure. When temperature of atmospheric air goes below dew point, water vapour present in atmosphere starts to condense into tiny water droplets over leaves of tree or over the grasses. These water droplets are called dews. 

At night, earth surface radiates heat and its temperature falls. In the early morning of clear night, temperature of atmospheric air goes below dew point as a result water vapour condenses in the form of dew (tiny water drops over the grass, leaves of trees). During cloudy night, cloud behaves as a blanket so that heatt radiated by earth remains trapped in atmosphere and atmospherie temperature cannot fall upto dew point. As a result dews are not formed.