Skis are a tool used with the [A]pply command. Wearing a pair of skis allows you to move quickly over the snow with slower gain of Fatigue. This is a critical winter-time advantage, especially when hunting. Animals are significantly easier to chase down, and NPCs may carry skis but do not appear to use them as of version 3.17. Skiing will also remove the effects of encumbrance on mobility, allowing you to move at full speed while carrying a heavy load.
Skis can be crafted from two boards, an axe, a knife, four pounds of animal hide and two pieces of tying equipment. This requires an axe and a knife, the perferred choices being the carving axe and the small knife.