How does adding salt ot the ice decrases its freezing point

Salt lowers the temperature at which water freezes and melts.  Pure water, 
H2O, freezes (and melts) at 0°C (32°F).  But the more "salt" (any elements 
or compounds carried by the water in solution, such as Na (sodium), Ca 
(calcium), Cl (chloride), and SO4 (sulfate)) in water, the lower its 
freezing point.  For example, seawater, which has approximately 3500 parts 
per million "salt" (including Na, Ca, Cl, SO4, Mg, K, and CO3), will freeze 
(and melt) at -2.2°C.  A water with extreme salinity such as very salty 
lake waters at Death Valley, California (approximately 300,000 parts per 
million "salt") may freeze and melt at temperatures as low as -20 - -30°C!
Because salt lowers the freezing point depression, it is added to icy roads 
in order to melt the ice.

The reason why salt lowers the freezing temperature of water is a bit more 
difficult to explain without discussing more complex chemistry.  Basically, 
pure water, H2O, is a different substance than salt water, such as NaCl-
H2O.  As different substances, they have different chemical properties.  
Salt "gets in the way" of the interactions between H and O, making it 
harder for the H and O to bond as ice.

You can do very simple processes in the lab or classroom illustrating this 
principle.  Fill two containers with water and put table salt in only one 
container of water.  Put both containers somewhere cold (in the freezer or 
even outside over night).  See what heppens!