Salt in chemistry refers to any chemical that results when an acid and base react. For example CaCl2, CaSO4, MgSO4, KNO3 are some salts commonly used in aquarium plant fertilizer.
However most think of is Sodium Chloride, common table salt. Farmers don't like sodium chloride since high levels can make the soil hard and unsuitable for terrestrial plants. At very high levels the ground would be as hard as rock and at medium levels it may make it hard for the roots to absorb nutrients and water. However in a an aquarium 10ppm of sodium chloride shouldn't have any effect on submerged plants. In fact I have seen drinking water reports with even higher levels listed.
i have looked at these products but have never seen a full list of what is in them. My best guess is that it is nothing more than river water that has been evaporated leaving behind all the mineral salts it contained. Without knowing what it actually contains it is not possible to say for certain if it is safe or not for plants, or fish. Also it is difficult to say if it has any therapeutic effect on sick fish. Salt is often listed as a treatment for sick fish but often it is not clear if they are talking about table salt or aquarium salt. My guess is ordinary table salt is good for fish since animals need sodium and iodine which is present in ordinary table salt and might be deficient in some tanks. Especially in tanks filled with RO or distilled water.