Low Intensity Cardio, or LISS, is Low Intensity Steady State training.
From looking at Kayla's blog post about the importance of LISS sessions, she believes you should achieve the best results by power walking for low intensity.
The idea behind Low intensity is to increase your heart rate to a level that is above your normal rate, and maintain this rate for a relatively long period of time.
Low intensity substitutes could include activities such as: yoga, swimming, slow jogging, cross trainer or cycling. Any form of cardio that is low intensity.
Kayla's BBG program recommends 2-3 low intensity sessions per week consisting of approximately 30-45 minutes.
You can read more about this type of cardio training in the Education section of both the BBG eBooks and the SWEAT app.