MSG is a flavor enhancer. It amplifies the Umami (Savory) flavor in food and is used to improve the taste of a variety of products, typically Junk Food but is most well known for its use in take out Chinese.

MSG has a bad reputation due to dubious scientific research done in the 1950s.

A single researcher published papers describing what he called 'Chinese restaurant syndrome' claiming that he always got sick from eating Chinese food and specifically blamed the MSG they used without any real hard evidence to back that up.

This researcher could have just as easily been getting sick due to an intolerance to soy, fish sauce, or any other commonly used product in Chinese cuisine.

Other research was performed on MSG in this era usually involved concentrations that were hundreds of times higher than used in food.

Later research showed that eating small amounts of MSG is perfectly healthy, and showed that his research was dubious at best and probably had racist underpinnings. But by that point the news media had caused it to go viral and caution against MSG has never gone away.