Fermentation in food processing is the process of converting carbohydrates to alcohol or organic acids using microorganisms—yeasts or bacteria—under anaerobic conditions. Fermentation usually implies that the action of microorganisms is desired. (Wikipedia, https://en.wikipedia.org/wiki/Fermentation_in_food_processing)
Simply it’s using microbes to naturally preserve, in our case fresh vegetables. While at the same time increasing the nutritional value or increasing many other health benefits.