How does MSG enhance food flavor?
As you are no doubt aware, there are 5 basic tastes - salty, sweet, sour, bitter and umami. Umami is the savoury flavour of mushrooms, cheese, cured meats, and so on. MSG is essentially 'pure' umami. In other words, MSG is to umami what salt is to salty and sugar is to sweet. So if you add it to savoury dishes (throw a parmesan rind in to vegetable soup while it's simmering) it enhances their savouriness.