HIIT is an acronym that stands for High-Intensity Interval Training. Intensity can be subjective to the individual. What may be intense for you may not be intense for someone else. HIIT training is giving 100% of everything you have for short periods. An example would be to full sprint for 30-40 seconds, then walking or jogging for 15-20 seconds. Generally, lasting for less than 30 minutes, or until failure. It is simply giving it all you have for a specified amount of time.