Contrary to some popular opinion, salt is an essential part of the diet. Unfortunately, it is possible to get too much of a good thing, and that is true in the case of salt.
On the other hand, it is possible to become deficient in salt, although that is quite rare with a modern western diet. However, vegans and other vegetarians can become deficient, especially when performing strenuous work in hot weather, since sweat is about 0.9% salt.