What is humidity?
Humidity is the amount of water vapor in the atmosphere. Air usually contains invisible water vapor, not to be confused with clouds and fog which are visible condensed water droplets.
Relative humidity is the ratio of the amount of water vapor in the air to how much it can hold. For example, if the relative humidity is 50% then the air contains half the amount of water vapor that it can hold at that temperature. The maximum amount of water vapor the air can hold is called the saturation point, and it increases with temperature.
Another measure of humidity is the dew point. This is the temperature at which water vapor in the air will begin to condense. The more water vapor in the air the higher the temperature at which it will begin to condense. When we see dew in the morning it's because the temperature dropped overnight to the point where the air became saturated with water vapor.
Humidity makes a hot day seem hotter because it slows the evaporation of sweat. Evaporation cools the surroundings. At 100% humidity the rates of evaporation and condensation of water are equal so there's no net evaporation and sweating is ineffective.