What is HIIT?

HIIT training is any workout that will alternate between short bursts of intense activity, perhaps 20 seconds, followed by a fixed period of rest, say 10 seconds. This series repeats for the duration of the workout.
High intensity cardio gets your heart rate up and creates an oxygen debt after your workout. This debt has to be paid and your body works hard to recover, which in turn, burns more calories. Your metabolism is increased and you are able to shed fat.