Like the wind chill, the heat index (or apparent temperature, "feels-like" temperature, or humiture) is another number to tell you how hot it feels on a summer day.
When you perspire, your sweat is evaporated from your skin, making you feel cooler. When the humidity is high; however, your sweat doesn't evaporate as fast, making you feel hotter.
The VERY complex formula for Heat Index:
Heat Index = (16.923 + 0.185212*T + 5.37941*RH - 0.100254*T*RH + (0.941695e-2)*T^2 + (0.728898e-2)*RH^2 + (0.345372e-3)*T^2*RH - (0.81497e-3)*T*RH^2 + (0.102102e-4)*T^2*RH^2 - (0.38646e-4)*T^3 + (0.291583e-4)*RH^3 + (0.142721e-5)*T^3*RH + (0.197483e-6)*T*RH^3 - (0.218429e-7)*T^3*RH^2 + (0.843296e-9)*T^2*RH^3 - (0.481975e-10)*T^3*RH^3)
Where RH is the relative humidity in whole numbers (74, not 74% or 0.74) and T is the temperature in °F.
Got all that?
We usually use a chart: