Dishwasher salt must be added to the water in hard water areas as it acts on the ions or particles in calcium and magnesium, (limescale) to soften the water. It is also used in soft water areas to aid rinsing. Dishwasher salt is used with a ionic exchange softener unit built into your dishwasher (if this is not in place there will still be an ionic reaction with the calcium which limits the amount of lime scale that will be built up). The salt used is sodium chloride which is course grained and prevents the softener unit clogging up. This is not table salt, adding table salt to your dishwasher will damage it by corroding the pipes inside it. You can buy this course grained salt at any supermarket (like the Sainsbury’s bag shown on the right).
If you have added detergent to the salt dispenser damage can be avoided by wet dry hoovering the substance and rinsing several times. Do not use the dishwasher until the dispenser is clean. The salt is generally added to compartment at the bottom of your dishwasher (a typical layout is shown in the image below).
If dishwasher salt is not used it is likely your dishes will come out streaky and gritty. It depends on the area where you live whether the water is hard or soft and therefore how much salt you will need to use. If you are in a hard water area you will find your dishes and utensils become cloudy with use and washing. The salt and the ionic reaction will prevent this.
Most modern dishwashers have an indicator light to remind you when to top up with salt. If not, there will be a float indicator which may be hard to see, you could make a regular day of the month to top up the salt in this case, just top up the dispenser until full.
Below is a video from Bosch which shows how to top up the salt reservoir and the process will be very similar for most other brands.