A water softener is a device that reduces the hardness of water by removing minerals like calcium and magnesium. It works by exchanging these minerals with sodium or potassium ions through a process called ion exchange. Softened water offers benefits such as preventing scale buildup in pipes and appliances, reducing soap scum, and improving the effectiveness of cleaning products. It is an effective solution for improving water quality and extending the lifespan of plumbing systems and appliances.