We described how adding salt (or anything) to water increases the boiling temperature. For a similar reason, it also lowers the freezing temperature. By adding salt, the liquid water in equilibrium with ice is not allowed to re-freeze, and eventually all of the ice is melted. The amount of salt used is important, as is the ambient temperature. If it is too cold the salt trick won’t work anymore.
Consider the curve we drew before, focusing on the region near 0 °C. In the zoomed-in version on the right, the vapor pressure of ice is shown along with that of 1) pure water and 2) water with some salt added (a solution). The H2O in question will exist in whichever state has a lower vapor pressure at a given temperature. The point where the curves intersect is the freezing point, and you can see how lowering the liquid curve with salt lowers the freezing point.
We use salt because it’s pretty cheap and safe and for other reasons like that. Other solutes like sugar or alcohol would work, too.
