A water softener reduces calcium or magnesium concentration in hard water. A water softener comes in two forms, either beads-filter or zeolite chemical-matrix-filter form. They work by replacing calcium and magnesium ions with sodium ions. Water softener fights to replace calcium and magnesium in the water, because they are exactly what are found in abundance in hard water. The ions dissolve easily in water, and due to cohesion, they tend to stick together. They also tend to bond with other substances, such as copper.
When a few ions bond with such substances, other ions will, in turn, bond with those substances which just bonded. These ions can cause problems with metal structures. The ions can build up as deposits inside water pipes and water heaters, and eventually can end up clogging the pipes, but can be broken down by water softener. In many households, calcium or magnesium may build up in cookware, especially so in coffee machines and kettles.
The clusters of calcium and magnesium ions forms what could be described as "scale," or, more informally, "buildup." Additionally, when one showers or washes their hands in hot water, the ions react with the soap to form a sticky "scum," which deteers away from the soap's ability to lather properly. Don't worry though, the problems with hard water can be solved, it can be treated to reduce the calcium and magnesium concentration. This is where water softener comes into play.