whats HIIT????
High intensity interval training, the most common way to do it is to sprint all out for 30 seconds, walk or jog for 15 seconds, then sprint again for another 30 seconds. If you have a track you can do 100m sprint, then walk/jog to the start of the next 100m and sprint again. In 4 laps you'll have sprinted/jogged a mile which is a serious workout.
You can pretty much do HIIT with anything though, as long as you give max effort.