Saltiness is one of the five primary basic tastes the
human tongue can detect. Those five tastes being: salt, bitter,
sweet, sour, and umami (it is from glutamic acid, which is found in
many foods, particularly some meats, and is the basis of the flavor
enhancer monosodium glutamate, also known as MSG).
Extra salt has other effects, beside simply making things more salty
it helps certain molecules in foods more easily release into the
air, thus helping the aroma of the food, which is important in
perception of taste.
Adding a bit of salt will also decrease the bitter taste perception
in food, which is why it is often sprinkled on grapefruit.
Salt does not suppress sweet or sour flavors as with bitter flavors,
but balances out the taste by making the perceived flavor of sugary
candies or lemons, less one dimensional.