H.I.I.T. stands for High Intensity Interval Training. When you break it down, you are basically working out at a very high intensity level, in very small time intervals. Many workout routines demand that you run on a treadmill for hours or perform dozens of reps repeatedly at a slow pace. HIIT is the opposite. HIIT requires you to work very hard, for a very short time. If you workout like this often enough, your body will burn calories in a different, more accelerated rate.