ELI5: How does salt enhance flavour?
The taste buds in the tongue are type of receptor called an ion gate. An ion gate is like a normal gate that can be unlocked with a key except the key is a metal ion of some kind, usually sodium or calcium.
Salt is sodium chloride, an abundant source of sodium ions. So basically the abundance of sodium ions keeps the gate unlocked and as a result the molecules that are the flavours are detected all the time.
Other users have answered how salt enhances flavor, but I'll answer why salt enhances flavor. You likely often hear that salt is bad for you, but that's not entirely true. Salt is actually just as important to life as water. All of your cells use dissolved salts (sodium, potassium, chlorine) to keep their cell membranes intact, run various cellular machinery, and communicate with other cells.
You are able to read this because your brain has billions of neurons constantly firing electrical impulses along complex circuits to process information. Each one of those electrical impulses starts with a neuron opening a gate and allowing sodium to flow into the cell and potassium to flow out.
A huge amount of your body's energy is devoted solely to carefully regulating the balance of water and salt in your body. Because salt was much harder to come by for our evolutionary ancestors, we evolved to love the taste of salt (to an extent). It's rare for modern humans to experience it, but we are actually wired to experience hunger for sodium if our blood sodium concentration gets too low.
All that said, don't get carried away and eat too much salt. The vast majority of people in industrialized countries are in no danger of salt deficiency, and too much salt can cause high blood pressure and other ailments.
Can I piggyback off this and ask about msg?
How does salt have a flavor itself?
It also makes you salivate more, which helps break down the food you're eating more effectively and therefore more taste