Que es un Heat Index?

So they keep saying the Heat Index will be around 110 degress today in NYC. I hear the term "heat index" all the time but I never really wanted to ask what it meant exactly. Actually, the more I think about hearing that term, the more I assume how terribly crappy it's going to be as I sit outside the campers today while everyone else is filming on the 6th floor. I'll let you all know how the "heat index" treats me.