The humidex is a computed value combining the temperature and humidity into one number. It is used to describe how hot or humid weather feels to the average person.
The number is only significant over 30 degrees celcius.
Essentially, on a really hot day, when there's signicant humidity, it generally feels way hotter than the actual temperature.
Holy crap, the humidex just hit fucking 40!!! It's too fucking hot to exist today!
(for those who live south of the border of Canada and the USA, 40 degrees celcius is actually 104 degree fahrenheit)