One simple reason why ice is slippery is that pressure and friction from shoes or skates can melt a very thin layer at the top of snow or ice.
It then forms a watery lubricant between ice and skate (good) or ice and shoe (bad).
Skis also glide on a layer of water.
The process by which ice melts under pressure and almost immediately freezes again when the pressure is off is called regelation.
