Rather than traditional ABS sidewalls, Slimewalls are made from urethane. That is the same material that is used in skateboard wheels, and it helps to smooth out the interaction between you and the snow.
For normal riding, it smooths out the little bumps you would get, and for park riding it helps to absorb big impacts from hitting rails and other tough things like logs and rocks.
On their Mobility range, they use 85A durameter urethane.
On their ATV style boards, they use 90A durameter urethane,
View Ride boards that have Slimewalls.