You have probably heard this term before but if not, in short it means short bursts of all out effort followed by a recovery period and then repeat. For instance, sprint on a treadmill for 30 seconds, then walk for 30 seconds and repeat for several rounds.
If your goal is to lose body fat, then 'HIIT' is the way to go. When you're working in the 'fat burning' zone, which is lower intensity for a longer duration, then yes, the majority of the total calories burned comes from fat. However, if you burn more total calories as you would with high intense training, the percentage of those calories burned from fat is lower but the grand total of calories burned from fat is actually higher because you burned more TOTAL calories in your session.
For example; 20 minutes of jogging on the treadmill with heart rate around 70% effort may burn 200 calories with which 50% of those came from fat giving you a total of 100 calories burned from fat.
Now, if you do 20 minutes of HIIT you may burn 400 calories with which only 30% came from fat giving you a total of 120 calories burned from fat. Lower percentage, but higher grand total.
As an added bonus, high intensity interval training has been proven to continue to burn calories in the 24 hours prior to finishing your workout. Steady state cardio will stop burning calories when you stop working out.
So work out smarter and get results faster by using your time more efficiently.