Dishwasher Salt is sodium chloride (e.g bog standard salt) with no iodine added sold for use in helping to soften water in dishwashers.
It is, however, a granular salt, with grains larger than table salt: the fine particulars of a salt ground for table use might clog the softener mechanism in dishwashers, or interfere with the process so that you end up hardening the water.
The soft itself doesn't soften the water. The salt doesn't even get in to where the dishes are. What it is used for is to "activate", or "reset" the mechanical device in the machine that does the water softening. The device is a basket-like contraption that contains resin in small yellow balls.
The purpose of the resin balls is to remove calcium and magnesium ions from incoming hard water. The resin has a negative atomic charge, and so attracts sodium ions (which have a positive charge) to its surface. Calcium and magnesium also have positive charges, but the calcium and magnesium ions are more highly charged, so they are able to elbow their way onto the resin, displacing the sodium ions on the resin in a 2 to 1 ratio: for every 1 calcium or magnesium ion that gets trapped on the resin, 2 sodium ions are released. By trapping the calcium and magnesium, the water is softened. The process is called "ion exchange."
The process only works, though, as long as there are sodium ions on the resin. After a number of uses, the resin becomes saturated with calcium and magnesium ions, and there are not enough sodium ions left to do the job of removing more from the water. The resin balls have be "reset" with brine (salt water.) A strong brine solution is allowed to flow over the resin, thus displacing the calcium and magnesium built up there, and resetting the surface with sodium ions. This is called "regeneration." Dishwasher machines usually do it automatically, either after a certain amount of water has flowed through or every several uses of the machine. The recharging brine is then flushed straight out, and doesn't even enter the dishwasher. Then, the resin balls are rinsed with clean water, and that rinse water is flushed out as well.
The water softener is at the bottom of each dishwasher. There's a lever or a front panel button on the machine that controls how much salt goes through. Some levers are easy to knock as you are filing the salt dispenser up with salt. The lever or button may be marked with a + / - . The numbers on the lever or button correspond to the "hardness" number for water for your area, which you can get from your local water authority, or the people who test your well for you. If it is set to minimum or 0, no salt, or practically none, will get through.
Some machines have indicators such as lights telling you when you need to top up with salt.
Dishwasher salt is commonly used in dishwashers in Europe, and in the UK. Sixty percent of the homes in the UK have hard water; most of these are in the south and east of England. The process is not needed in Scotland, however, as the water is very soft there.
When you go to fill the salt container, it may be full of water. Just put the salt in anyway; it will sink to the bottom and displace the water. Besides, when the salt gets used, it gets made into a brine solution anyway, so don't worry about needing it dry. Most machines come with a funnel, that you stick into the hole in the salt cup to make it easier to fill.
If you live in a hard water area, you need to use salt even if you use 3-in-1 tablets, which contain salt -- as that salt doesn't reset the water softener device.
Watch out if you have North America relatives trying to help in the kitchen, so that they don't make the mistake of putting detergent in the salt compartment.