Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100% relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain, but the relative humidity near the ground could be much less.
Humans are sensitive to humidity, as the skin relies on the air to evaporate moisture. The process of sweating is the body's attempt to keep cool and maintain its current temperature. If the air is at 100% relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off. For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius) and the relative humidity is zero percent, the air temperature feels like 69 degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75 degrees Fahrenheit (24 C) and the relative humidity is 100 percent, it feels like it's 80 degrees (27 C) out. Humans are normally most comfortable at about 40% humidity.