Not so complex. wet bulb refers to a thermometer with a wet piece of cloth over the bulb. The rate of evaporative cooling causes the thermometer to read cooler than the air temperature. This temperature is compared to the air temperature (using a "dry bulb") under a standard procedure (whirling the pair of thermometers over one's head). The difference between the wet bulb and the dry bulb indicates the relative humidity by way of a table or graph.
The deadly wet-bulb temperature of 95-96F corresponds to a 95F day with 100% humidity, or a 110F day with about 65% relative humidity, or 120F at 40% humidity. Supposedly they have come close to these values already in the Persian-Gulf states - which can experience severe heat and humidity. Countries like Bahrain, Quatar, and the UAE and maybe parts of India will become uninhabitable outside of air-conditioned spaces some times of year in the next few decades. Any but brief chores outdoors work will require some kind of air-conditioned space-suit.
Complex warm-blooded life can adapt to extreme cold far more easily than extreme heat - just look at the diversity of mammalian and avian life in the arctic compared to the Arabian peninsula. This is something that climate denialists miss when they say "climate has always changed". Yes, it has changed. But, setting their ignoring of the rate of change aside, the global climatic excursions in the past have generally been toward colder temperatures than today, not hotter temperatures. The hottest spells have have been always associated with mass extinction, the coldest spells, with diversification of complex life.