Fill the salt? I'm not sure all dishwashers have a receptacle for that. Ours doesn't, and I'm pretty confident that I've never had one that did across several states in the US and one place in Belgium.

Is the salt for doing a softening process that substitutes for a whole house softener if you don't have one?

We live in a place with hard water and have no softener in the house (you should see our electric kettle when we haven't cleaned it recently). Recently the accumulation of what I assume to be mineral film has gotten a bit noticeable on the dishes that go in the dishwasher.