This is talking about the weather conditions at which a person is guaranteed to die if they are outside for six hours in the shade (or at night).

Previously climate scientists said this would happen at a wet bulb temperature¹ of 35 Celsius, theoretically enough to prevent a sweat-drenched human body from overheating. However, research has demonstrated the threshold is lower and doesn’t perfectly follow a single wet bulb temperature. And the scientific article that the news article is about shows these conditions have already occurred several times, when it was previously thought this threshold had not been breached yet.

Of course people can find shelter in an air conditioned buildings, underground, or under a forest canopy. But billions of people do not have access to these options. At some point they can either die or migrate, and this research shows that point requires less climate change than previously predicted. Combined with climate change occurring at a faster rate than the median expectation, mass climate migration is coming a lot sooner than expected.

¹: the temperature a thermometer indicates if the bulb is wet. If the air is dry, evaporation will cause this temperature to be lower than the air temperature, which is also the temperature a thermometer indicates if the bulb is dry.