Humidity is a measurement of how much water vapor is in the air. However, it is good to remember that warmer air holds more water vapor than cold air. Meanwhile, relative humidity is a measurement of how much water can air hold at a given temperature.
Once relative humidity reaches 100%, air starts to release the water it can’t hold. Later on, the water will condensate to fog, morning dew, or rain. The relative humidity for plants can vary. Plants that grow in hot, tropical areas adapt well to higher relative humidity.
As a rule of thumb: Indicas prefers a colder, less humid environment, while Sativas prefer a warmer and more humid climate.