Ajinomoto is a Japanese brand name for MSG, monosodium glutamate, a flavor enhancer. Ac'cent is the primary U.S. brand name, and the Chinese version is called ve-tsin.
MSG has very little flavor of its own, but it 'improves' or enhances the flavor of other foods it is used with.
Some individuals seem to be sensitive to MSG, and exhibit what is known as 'Chinese Restaurant Syndrome' (because of its use in Asian cooking). The symptoms include headaches, chest pains, facial pressure, burning sensations, and sweating.
The FDA continues to list it as GRAS (Generally Recognized As Safe), but all foods containing MSG must state so on the label.