Glutamic acid is an amino acid, and it's a natural part of proteins found in many foods. It's fine when it's contained in proteins, but it can be converted to a salt called Monosodium Glutamate, which can be added to food to give it a savory flavor. It's also present in other ingredients, such as yeast extract or hydrolyzed protein, so it's not always obvious when food contains MSG.
MSG excites the nervous system, and many people are sensitive to it, causing them to feel anxious, hypersensitive, have panic attacks, or have even seizures. Since it isn't always obvious when food contains MSG, these people have a very hard time avoiding it. The FDA could require food manufacturers to list the amount of MSG in food, regardless of its source. This would be helpful for sensitive individuals.