Condensation of atmospheric water.
As sets temperatures drop. The absolute humidity (actually mass of water vapor in the air) remains constant but because cooler air cannot hold as much vapor as warm water the relative humidity (percentage of the total amount of water vapor the air can hold versus the absolute humidity). As the temperature reaches the dew point ( the temperature where relative humidity reaches 100%) condensation starts to occur. This can happen a number of ways but for dew to form the condensation has to occur on objects on the Earth, often grass)
For frost to occur the same process happens but the temperature of the condensation surfaces needs to be below freezing, in which cause the water freezes onto objects, which is called frost.
