Limescale (also known as scale) is an off-white, chalky substance that tends to build up around plumbing. You've probably noticed it around your faucets or showerhead. It's mostly made up of calcium and forms of salt left behind when hard water evaporates. Sometimes it will combine with soap, which normally dissolves in soft water.
Limescale isn't harmful to humans, but no one wants to drink it. It can affect the taste and definitely the look of drinking water.
If your house has hard water running through its plumbing, limescale can build up along the inside of the pipes. Some refrigerator water filters can remove limescale, though refrigerator manufacturers don't often rate them for it. To be sure that limescale is removed from your drinking water, you can use either in-line or undersink water filters by Aqua-Pure.