A water softener is a box that is plumbed in to your incoming water supply that removes the limescale. (and a few other things)
It does it by collecting the limescale etc on resin beads that are automatically "re generated" i.e. flushed by a salt solution, usually run overnight.
Cost of machines with plumbing is around £450 to £1,500 depending on features and capacity etc. Our cost of salt is around £90 per year.
The water tastes much the same, but can have a slightly elevated sodium content in theory, but it's still around six times less than milk. Some people leave a "hard" tap for drinking, but I haven't bothered and certainly can't taste anything.
Our taps and shower screens rarely need cleaning, and never descaling.
Popular brands include BWT, Harveys and Ecowater.
You probably will need a plumber to fit it, and a suitable place to put it with power and access to a drain (which it needs for the regeneration process).