A thing is slippery when its resistance to motion along its surface is very slight. This resistance, or friction, is a thing one rather dislikes – especially in machinery. But one should find things very inconvenient without it. Walking and running would be utterly impossible without friction, and even standing would require most perfect balancing. When we walk on ice, we get as much friction as possible from our footwear. If we tried to walk with shoes soled with ice, or covered with even layer of oil, we could not walk at all, for there would be particularly no friction to keep our feet where we planted them. Some people use a shoe-grip (as seen in the photo below) to walk comfortably on ice. The absence of friction in the case of ice depends upon its beautifully even, crystalline structure.
The molecules of water are held together very smoothly and evenly, and this is especially so if the ice was formed when there were no currents in the water and no wind, so that its surface was very smooth when it became frozen. Various substances, like oil and wax and varnish, give an extremely smooth surface even to things like wood, and so render them almost free from friction.