Very simply, this is a measure of temperature that accounts for humidity.
From an article entitled Why you need to worry about the ‘wet-bulb temperature’:
Wet-bulb temperature (WBT) combines dry air temperature (as you’d see on a thermometer) with humidity – in essence, it is a measure of heat-stress conditions on humans.
It’s named for how it’s measured – a wet cloth is held over the bulb of a thermometer (or something, I wasn’t quite sure).
Wet-bulb temperatures are more extreme that “normal” temperatures. From Wikipedia:
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.