Water softeners treat hard water by using ion-exchange resins and salt to remove magnesium and calcium hardness from the water. These resins are coated in a sodium solution and sodium ions migrate to the active sites in the resin out of the solution and replace in the solution the magnesium and calcium ions, when hard water comes into contact with all the resin beads.
Hard water is softened by removing magnesium and the calcium it contains.When hard water passes through the softening system's resin bed,it swap the tough ions in the water for ones that are soft ,so just softened water passes through to your home.