Salt is spread on icy roads to make them less slippery and safer to drive on. It does this mainly by changing how water freezes and melts.
Freezing point depression
Salt dissolves in the thin layer of liquid water that exists on the surface of ice, even when it is at the freezing point. This creates salty water (brine) that has a lower freezing point than pure water, so the ice starts to melt and is less likely to refreeze at typical winter temperatures. Because of this, roads can stay wet or slushy instead of being covered in solid, slick ice.
Improved traction
As ice melts into salty slush, tires can grip the road surface better than on hard, smooth ice. Undissolved salt grains on the surface also add a bit of roughness, further improving traction. A road with salt-induced slush is therefore generally safer to drive on than one coated with a continuous sheet of ice.
Temperature limits
Salt only works well down to a certain temperature; ordinary road salt (sodium chloride) becomes much less effective in very cold conditions. At lower temperatures, road crews may switch to other salts like calcium chloride or to sand, which does not melt ice but helps with traction.