This is something figurative. Nobody throws salt into any wound. Let's say you are mad with your boyfriend, and someone is telling you bad things about him... What this person is doing is making your feel worse, then, he is throwing salt into your wound.

Suponga que "yo" tengo ganas de echarle sal a la herida de "él". Se podría decir, "I'd love to pour salt on his wound" o "I'd love to rub salt in his wound."

This sentence is bad formed, the correct form is "que ganas de echarle sal a la herida", and it means some like "I have desire to throw salt into the wound" Sorry for my english jaja :)