Regeneration is the process of restoring the softening ability of the
resin which is inside the water softener unit.
Water is softened in an ion exchange process where sodium ions
are exchanged for calcium and magnesium. When the sodium ions are all
used up, the softening resin becomes exhausted so the water softener then 'regenerates' the resin with brine (salty water) and the calcium and magnesium are
exchanged for sodium.
The resin is then flushed with water and cleaned back to its
original softening state.