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