MSG stands for monosodium glutamate. It is supposed to bring out "meat" taste. There has been a great debate about it. While generally it is felt to be "safe", there is a powerful lobby behind it.
Glutamate is a powerful neuro transmitter and although MSG is a imple amino acid, it does have potent biological properties. Those who may have "allergies" might have earlier adversely affected by MFG. After consuming massive quantities of MSG for over a period of a month, I became terribly depressed. While there is no scientific literature, be careful in consuming daily/routine/large doses of MFG.