Why is ice slippery?
The short answer
Molecules at ice's surface are more mobile than those inside, and pressure or frictional heat can contribute to a very thin water layer.
How it works
Slipperiness depends on temperature, roughness, and the material touching the ice. It is not explained by one fixed water layer in every situation.
A familiar example
Treads can help move water or snow away and change contact with the surface.