Relative humidity is a measure of the amount of moisture in the air relative to the total amount of moisture the air can hold. For instance, if the relative humidity was 50%, then the air is only half saturated with moisture. Warmer air can hold more moisture than colder air.
Without humidity, there would be no clouds, no precipitation, and no fog. Humidity is simply water vapor in the air, which is needed to form rain. Additionally, water vapor holds heat in the air. It is a greenhouse gas, which means it is a can absorbs heat and warms the atmosphere. This is why humid air feels warmer.
Relative humidity is reported as a percentage of the total amount of moisture that could be held in the air. For instance, if the air is only half saturated with water vapor, the relative humidity will be reported at 50%. If the air holds all the water vapor it can hold, it is 100% saturated.
A capacitance-type sensor is one way to measure relative humidity. Changes in relative humidity cause the shape of the sensor to change. When the sensor changes shape, it’s capacitance, or ability to store an electrical charge, changes in a detectable and repeatable fashion.