Salts are typically forms of fertilizer that are water soluble and are available immediately for the plants to use. Many chemicals come in a few forms, such as their base form which is often not water soluble. By changing the chemical's form, it can be easily used by the plant. Salts, like table salt, mess with water as shown in the example with a bunch of grass in salt water. Salt attracts water for the most part, and it is responsible for helping and maintaining the water pressure inside the plants. If there is too much salt, the salt pulls water out of the roots, killing the plant in the process.
Plants need very little nutrients to grow. Luckily, the amount of water based nutrients/salts is not enough to affect the plant unless it is overdone. When this happens, the plant will wither, looking like it needs watered, and the nutrient (lest say Nitrogen,) will be too strong for the plant, also causing a type of nutrient burn. FWIW, a 16 Oz tub of the fertilizer I use will last me at least 5 grows. I'm not saying that this is all that I feed them, but I don't feed them unless they look like they need it, which comes from getting familiar with your strains and setup.