Since this has been coming up a lot lately,
The wet-bulb temperature is the temperature read by a thermometer covered in water-soaked cloth over which air is passed.  The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.

Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32°C (90°F), equivalent to a heat index of 55°C (131°F). A reading of 35°C (95°F) -- equivalent to a heat index of 71°C (160°F) -- is considered the theoretical human survivability limit for up to six hours of exposure.
In other words, if the wet-bulb temperature gets too high, the human body loses its ability to cool down by sweating, so you can simply die from being too hot. #science