Wet bulb is part of a thermometer pair: wet bulb/dry bulb. The dry bulb will give you the absolute temperature of the air and the wet bulb will give you a cooler temperature based on the cooling effect of the evaporating water on the bulb. The drier the air the cooler the wet bulb, since there will be more evaporation. There is a formula for determining the relative humidity based on the wet bulb and dry bulb temperature readings.