Simply stated, Relative Humidity (RH) is the amount of water vapor present in the air expressed as a percentage of the amount of water vapor needed to saturate the air at the same temperature.

It is a way of converting the actual amount of moisture in the air into a simple value dependent on the temperature of the surroundings.

Basically, the more water vapor that is in the air, the higher the RH will be. Similarly, if there is less water vapor in the air, the lower the RH will be.