Humidity is moisture in the air. There are several measures of humidity. One is absolute humidity which is the greatest amount of water vapor the air could hold at a certain temperature. Relative humidity is how much actual water vapor is in the air at a certain temperature. Thus humidity is expressed in terms of percents.
Humidity is measure with a hygrometer.
The rate of evaporation goes down as the humidity in the air increases - there is not enough room in the air to take more water vapor through the process of evaporation.