"Saltiness” is one of the five primary basic tastes the human tongue can detect. Those 5 tastes are: salt, sweet, bitter, sour, and umami (in case you don't know this word, it is from glutamic acid, which is contained in many foods, particularly some kind of meats. It is the basis of the flavor enhancer monosodium glutamate, also known as MSG).
The salt that we add to food has other effects except making it saltier. Adding salt to food, helps some molecules contained in those foods to be released to the air, helping the aroma of the food to be richer, which is important in our perception of taste.
When we add salt to sweet or sour food, will help balance out the taste by making the perceived flavor, for instance of sugary candies or lemons, less one dimensional.
Salt also has been shown to help suppress the bitter taste. So adding a bit of salt to bitter food (i.e grapefruit) won’t just increase your salty taste perception, but will also decrease your bitter taste perception in any given food