High-intensity interval training (or HIIT for short) means exerting your body to its fullest for a short period of time, recovering, and then going all out again. For example, in a HIIT workout you'll sprint as fast as you can for a minute, rest for three minutes, and then repeat the cycle. This form of exercise only needs about 20 minutes, so it's perfect for busy folks who don't have time to hit the gym.
HIIT is a fast, effective way of increasing your fitness and losing weight. As the name implies though it is intensive, so HIIT is not recommended for beginners or people who haven't exercised for a while. Build up to it, and then incorporate HIIT into your fitness regime. You will see remarkable results in a short amount of time.
Please also note that there are all sorts of workouts and exercise; some are rigorous whereas some are slow-paced. As each workout or exercise is of different intensity, it is important to try the workout according to your fitness level. If you are unsure about proceeding with the suggested workouts, please consult your physician or other healthcare professional before beginning or changing any fitness program to make sure that it is appropriate for your needs.