HIIT stands for High Intensity Interval Training.
HIIT training could include; sprints, cycling, skipping or using the cross trainer. Any form of cardio that you can make high intensity.
HIIT does not appear within the BBG programs recommended training until Week 9.
Weeks 1-8 have been designed to prepare your body for HIIT, and an increase in the intensity of exercises from week 9.
For more information on HIIT training check out the Education section of the BBG program or head to Kayla's blog.