Ice forms when the temperature of water reaches 0 degrees Celsius. Road salt works by lowering the freezing point of water via a process called freezing point depression. The freezing point of water is lowered once the salt is added, so the salt makes it more difficult for water to freeze. For example, A 10 percent salt solution freezes at -6 Celsius, and a 20 percent -16 Celsius.
To note that solid salt alone can’t get into the structure of the frozen water to start the dissolving process, so there has to be at least a tiny bit of water on the road for freezing point depression to work. That’s why you often see trucks pretreat roads with a mixture of salt and water.