The amount of water vapor in the air is referred to as humidity. The more water that is in the air there is, the higher we say that the humidity has risen.
Humidity can be measured in a number of ways. These include absolute humidity, specific humidity, and relative humidity. Absolute and specific humidity measure the exact physical amount of water in the air, while relative humidity compares how much water is in the air with how much could be in the air, or in other words, with the maximum vapor pressure.
Suppose that given the local temperature and wind, the maximum vapor pressure allows a certain volume of air to hold 100 pounds of water. Now suppose that this same volume of air is only holding 30 pounds of air. The relative humidity would be 30%, because only 30% of the maximum vapor pressure has been reached.
Thus, relative humidity does not tell us how much water vapor is in the air, but rather, what percentage of the maximum vapor pressure has been reached.
Relative humidity is the most common way that we measure humidity.