Long before we had fridges, fermentation was used to preserve goods to give them a longer life. Fermentation is when bacteria and yeast - microorganisms - break down carbohydrates such as starch or sugar and turns them into alcohol or acids. This acts as a natural preservative and gives the foods a stronger, tangy taste. Fermentation also creates good bacteria, which help to promote a healthy gut.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/kimchi-and-sauerkraut-what-are-the-benefits-of-eating-fermented-foods