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.
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 the BBG program.