The short answer

Vinegar is an acidic liquid that occurs when you let fermentation go farther than wine. Vinegar preserves food by raising the acidity of the food, causing microorganisms to die instead of rot the food.

The long answer

Legend has it that vinegar was first discovered after someone forgot about wine for a couple of months and found it turned sour. This is why the word "vinegar" derives from the French “vin aigre,” or sour wine.

Vinegar is a combination of water and acetic acid produced with a two-step fermentation process. To make vinegar, you first ferment a plant food, like fruit, whole grains, or potatoes, by letting yeast feed on the plant sugars until the liquid turns into alcohol.

Once you have alcohol, you then ferment it further by exposing it to oxygen which allows naturally occurring acetic acid bacteria to eat the alcohol and produce acetic acid. While not strictly necessary, people often speed up the second fermentation step by adding a "mother" which is a biofilm that is created as a byproduct of turning alcohol into vinegar.

Vinegar has been used as a preservation tool for thousands of years. When food is submerged in vinegar (i.e. pickling), the acetic acid solution raises the acidity of the food and kills off any microorganisms that would otherwise cause the food to rot. It essentially slows down the decaying process but doesn't stop it – vinegar preserves food for about 5-6 months.

Sources

Hansen, S. (n.d.). Can Fruit Ferment On Its Own? Learn The Facts Of Fruit Fermenting. HomeBrewAdvice. Retrieved December 27, 2022, from https://homebrewadvice.com/fruit-ferment

Harvard T.H. Chan School of Public Health. (2019, October). Vinegar. The Nutrition Source. Retrieved December 27, 2022, from https://www.hsph.harvard.edu/nutritionsource/food-features/vinegar/

What Is a Mother of Vinegar, and Where Can I Find One? Revolution Fermentation. (2022, August 3). Retrieved December 27, 2022, from https://revolutionfermentation.com/en/blogs/fermented-beverages/what-is-mother-vinegar