And in a general way we understand what that means.
Humidity refers to the moisture content of the air.
If the air is dry, then perspiration evaporates rapidly. This cools our bodies because evaporating water requires heat, and that heat is removed from the surface of our skin. If, on the other hand, the air is very moist, then perspiration tends to cling to us without evaporating and we lose the benefit of this cooling. Air at a particular temperature can hold only so much water. Air holding the maximum possible amount of water is said to be saturated .