A slug has quite a bit of water inside of it, and the cells that make up its skin have highly permeable membranes. When you sprinkle salt on a slug, it mixes with the water in the mucus that the slug secretes to help it move around, creating a salt-water solution. That solution has a higher salt concentration than the inside of the slug, so osmosis occurs and water from the slug’s skin cells passes through the membranes to dilute the solution and even things out. If you use enough salt, the slug will lose so much water that it dehydrates, dies, and winds up looking pretty shriveled.

Humans can handle salt without the same thing happening because our skin isn’t as permeable as a slug’s. Put some salt in your eye, though, and you’ll get a small sense of what the slug is going through.