Dishwashers need salt to prolong their life by stopping limescale (calcium carbonate) from forming on the heating element and to keep internal parts generally clean.
If you use dishwasher powder and rinse-aid liquid, you need to add special dishwasher salt granules (not ordinary salt) to the water in the base of the dishwasher (under the screw-on cap). Buy them where you buy the powder.
If you use dishwasher TABLETS they usually have salt included. (Check the ingredients.) If so, you do not need to use salt granules as well in this area of the country. If you do, the cups and dishes will get too much salt and come out tasting salty.