Fermentation has been practiced by cultures around the world for centuries to preserve the harvest and increase the digestibility of fresh foods. During the fermentation process, naturally-occurring microbes convert the sugars and starches in raw foods into lactic acid, acetic acid, carbon dioxide, and sometimes alcohol.
For general information on fermentation, here are some resources we've found helpful:
Katz, Sandor Ellix. Wild Fermentation: The Flavor, Nutrition, and Craft of Live Culture Foods.