Facts about Hard Water
The water from our taps is not pure. Water is a good solvent and picks up impurities easily. Water described as "hard" is high in dissolved minerals, specifically calcium and magnesium. Hard water is not a health risk and can actually have health benefits by providing vital minerals. Some homes in hard water areas have a dedicated water softener, but the softened water is not suitable for drinking due to its high sodium content.
Hard water can be a nuisance because of mineral build-up on heating elements of household products - thereby affecting their performance and service life. Hard water can also affect detergent performance. A simple way to see if you live in a hard water area is to look in your kettle and see if there are deposits on the heating element.
The map from the DWI opposite indicates regions of the UK and the hardness of water supplies.
Your dishwasher features a dedicated water softener that helps prevent limescale but it does need to be topped up with dishwasher salt periodically. Most machines have an indicator that tell you when the salt and rinse aid need replenishing.
If you haven't been using dishwasher salt or descaled your dishwasher for a while, you may want to consider removing the existing limescale.