A 2018 study found two kinds of molecules, the tiny particles of water, on the surface of ice. Some stick to the solid ice below. Others, held by fewer links, roll over the surface like tiny balls.
Sliding objects glide over this tumbling layer, making ice slippery.
Ice was slipperiest near minus 7 degrees Celsius. Colder than that, fewer molecules roll, so friction rises. Warmer, the ice softens, so sliding objects dig in.