Salt is added to icy roads in the winter because it lowers the freezing point of water, which also makes the salt melt the ice. A small amount of water must be present on the roads for it to melt the ice.
When the salt is added to the ice, it first dissolves with the thin layer of water. This layer lowers the freezing point of the water, so more ice melts. When additional ice or snow falls on top of the salty water, it melts. There is a limit to the amount of salt that can be dissolved, so when it reaches the maximum level, it will stop melting the ice. New layers of salt must then be added.