The concentration of water in the air is constantly changing. As liquid water evaporates to water vapor it enters the air and increases the concentration of water in air. The water vapor in the air condenses to water and falls out as liquid leading to a decrease in the concentration of water in the air. The rate at which these processes occur is dependent on many factors including temperature, pressure and the total amount of water in unit volume of air.
Dew point refers to the temperature at which the rate of conversion of liquid water to water vapor is equal to the rate at which water vapor is converted to liquid water. If the temperature of air is lower than the dew point, water vapor condenses and leaves the air in the form of water. At this point relative humidity is essentially 100%.
The dew seen of leaves in the morning is the result of atmospheric temperature falling below the prevailing dew point. If the concentration of water in the atmosphere over a large volume is high enough, there is a possibility of rain when the temperature of air falls below the dew point.