This is the air temperature measured by a mercury-in-glass thermometer which has the mercury bulb wetted by gauze that is kept moist by a reservoir of water.
When exposed to the environment the moisture evaporates from the wetted gauze, which gives a lower reading on the thermometer.
This gives an indication of how ‘dry’ or how ‘moist’ the air is, since in ‘dry’ air the water will evaporate quickly from the gauze, which depresses the thermometer reading.