Humidity (adjective: humid) refers to water vapor in the air. It’s not the rain or fog or clouds, but moisture that’s in the air around you that you can’t see.
Deserts usually have low humidity, and tropical regions have high humidity. The word “humid” often means that the humidity is high, which gives the feeling of being very damp (wet air), stuffy, or even sweltering when the temperature is also high. When the humidity is low, the air can be said to be “not humid” or “dry.”
Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin.
Humidity affects the rate of evaporation. With a higher humidity, the rate of evaporation is less. The air is already full of water so it’s harder to squeeze more in.
(Edited from source)