Heat index it's a measurement of how hot it really feels outside, considering both air temperature and relative humidity.

The temperature outside feels hotter as humidity rises. This happens because in a dry environment our bodies are able to cool themselves better. When it gets hot on a dry summer day, our bodies produce sweat. That sweat captures the heat energy and evaporates, taking the heat with it. Conversely, as you sweat on a humid day, the sweat cannot evaporate as much because there is already moisture in the air, and therefore less heat energy is transferred off your body and into the air.