There are 2 types of water softeners being promoted as salt-free or saltless.
There first type uses potassium chloride instead of sodium chloride to recharge the resin that softens the water. Sodium choride (or what we know as table salt) is used in regular water softeners. The downside of this is that it adds sodium into the water making less desirable to drink. Plus it is not very good to the environment when flushed out into the drainage system. Potassium chloride is technically also a salt. But because it is not the table salt we eat, the marketing departments of the water softening industry thought it would be a good idea to call these no-salt water softeners and get away with it. Sure, now sodium is no longer introduced into the home water plumbing, but potassium isn't that great either. Plus it is about 5 times more expensive to replace the salt now and it is not as efficient as regular salt
The other type of saltless water softeners are not really water softeners in the sense that we understand water softening. They are better known as hard water conditioners.