LISS stands for Low Intensity Steady State.
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 LISS.
The idea behind LISS 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.
LISS 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 LISS sessions per week consisting of approximately 30-45 minutes.
You can read more about LISS cardio in the Education section of both the BBG program and the SWEAT App.