What is the heat index, and how does it vary from air temperature?

ROCHESTER, N.Y. (WROC) — Throughout the summer, much of the country has experienced at least a few days of extreme heat.
In some cases, Americans were seeing large differences between real and 'feel-like' temperatures. In Rochester, New York, for example, high temperatures were only reaching the mid-90s during a multi-day stretch of heat, but the 'feel-like' temperatures were nearing the triple-digits.This 'feel-like' temperature can be identified as the heat index, but how could the heat