In the context of snowsports Piste refers to a well packed snow trail that has been groomed, normally by machine.
Skiing on-piste is generally easier than skiing off-piste because the grooming that occurs smooths out bumps and potholes. It also packs the surface down providing a firmer base.
In many circles, it is considered cooler to ski (or snowboard) off-piste. This is because skiing off-piste requires far more experience and skill. Off-piste is also off the beaten track, far away from the maddening crowds.
Usage:
"Forget that bitchass on-piste crap. I go off-piste all the way baby!"
French, from Italian pista, to trample down.