The freezing point of water, the temperature just cold enough to make it freeze into ice, is 0 degrees Celsius. Dissolving salt in water makes the water harder to freeze. To get salt water to freeze you have to make it colder than 0 degrees. So you can say that salt lowers the freezing point of water.
If you add salt to ice, some of the ice will melt. You can suppose that the salt pulls some water away from its crystal form in ice. When that happens, the ice-salt mixture will get colder than 0 degrees.Here is a recipe for making a very cold freezing mixture: Mix 33 ounces of salt with 100 ounces of snow or finely crushed ice. That is supposed to give a temperature of minus 21 degrees C.