HIIT stands for "High Intensity Interval Training". If available for your program, you will have a HIIT Guide button under the "Supplemental Workouts" section. This type of exercise is designed to boost calorie burning by raising your heart rate high for a short period, then offering a longer rest period between intervals.
To use: You can pick from any HIIT exercise when your workout guide specifies you can add HIIT on that particular day. Simply go into the HIIT Guide button (if available), and pick the one you want. You will see a description of exactly what HIIT exercise to do, with number of reps and intervals included.
Please see example below of where the HIIT Guides are located in Ripped in 90 Days and Fat Loss Extreme for Her.
To incorporate HIIT into your workout: