At freezing temperature of water, molecules enter and leave the solid at same rate; but adding salt to it disrupts it. Adding salt to the cold water causes the freezing point to become lower. Additionally, when ice melts, it gets energy from the surrounding liquid and the liquid will cool down. The temperature makes the water molecules slow down enough so that more can be attached to ice.
If the concentration of the salt is at high point, the lower the temperature of the freezing point.