HIIT stands for High Intensity Interval Training.
HIIT training can include sprints, cycling, skipping or using the cross trainer. Any form of cardio that you can make high intensity.
The idea behind HIIT is to significantly raise your heart rate for a short period of time before allowing a brief rest period and to then repeat this cycle for a certain time or distance.
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.