Humidity is the concentration of water vapor (water when it's a gas) in the air. Some places have more and make you feel soggy and hot like you're in a sauna. Other places don't have much, which makes it easier to get nose bleeds and static zaps. These are just a few examples of how humidity can have a noticeable impact on things that grow, like mushrooms or humans, as well as things that are sensitive to moisture, like delicate electronics, or humans.