The ions from the salt make it more difficult for the water molecules to form hydrogen bonds with each other. Water molecules in the solid state have
Water molecules in the liquid state bond to approximately 3.4 other water molecules. When water molecules are in the solid state, each water molecule forms hydrogen bonds with 4 other water molecules. This locks the water molecules in place and do not move, like children who have been tagged in a game of freeze tag.
Salt will depress (lower) the freezing point of water. The more salt you add, the lower the freezing temperature of the water will be. See how to calculate this by watching the video below.
There is a really nice discussion of this topic on General Chemistry Online .
Hope this helps!