What is this option?

idefix37

It gives the wet bulb temperature, the temperature provided by a thermometer whose bulb is wrapped in gauze and constantly moistened. This thermometer is always present in a traditional weather station (not electronic). It allows to calculate the relative humidity.
But in Windy that’s not the aim as RH is already available.
Wet bulb temperature gives a good idea of the discomfort of high temperature with high humidity. And when the wet bulb temperature is reaching levels close to the human body it means that you are no more able to cool your body by sudation. It can lead to death. This parameter is useful in hot and humid conditions.
BTW to get information about a layer, click the small i in bottom right corner of screen of the browser version or About this data in mobile app.
More information: https://theprint.in/environment/what-are-wet-bulb-temperatures-and-why-they-probably-wont-cross-35c-long-enough-to-be-lethal/951637/?amp