Nicotine salt is the form of nicotine found in tobacco leaves. Unlike the nicotine ordinarily used in e-juice, these salts aren’t just nicotine; they’re compounds containing nicotine and another organic components. This could be in combination with one or more of the other chemicals in tobacco leaves, but the result is the same – a more stable molecule is formed.
The form of nicotine ordinarily used in e-liquid is called “freebase” nicotine. This is nicotine not bonded to anything else, and is generally considered preferable for vaping and other purposes because it’s a more “volatile” (i.e. easily vaporized) form of nicotine.
In fact, the tobacco industry went to considerable effort to increase how much nicotine is in a cigarette. A major contributing factors to the success of Marlboro cigarettes, because Philip Morris was the pioneer of this successful approach.