Humidity is the amount of water vapor (water that is a gas instead of a liquid, usually because of evaporation) in the air. There are different types of humidity that can be measured in different ways. Some measurements can be used to predict rain and snow. A lot of fog in the air is one result of high humidity.

This is an image taken from a satellite of water vapor moving across North America. Humidity is the measurement of the amount of water vapor in the air.