Off-piste skiing means to descend any slope that is not marked by piste or trail marker poles and has not been prepared by piste grooming machines, which smooth-off the surface of the snow.
Ski pistes, or prepared ski slopes, have poles on each side marking the pistes, and are patrolled by the ski patrol. Pistes are generally groomed each night by grooming machines which prepare the snow for skiers the next day. This makes the skiing easier than if the snow had been left in it’s natural condition. Skiing on these kind of slopes is called on-piste skiing.