Why do they put salt on the road before its snows?

Salt added to ice will lower the freezing point of water. Thus water, which normally freezes at 32F, with salt added will freeze at a lower temperature. So salt doesn't actually "melt" the ice and snow, it just allows the water to exist as a liquid near its freezing point rather than a solid.

Ice is solid water-vapor water system in equlibrium. Adding salt to the system disrupts the equilibrium by increasing the number of particles in the vapor. So the ice melts to re-establish the equilibrium of the vapor. The idea too is that the added particles disrupt the solid water bonds, where the water and salt dissolve into one another, creating a solution with a lower freezing point.