Before refrigeration and preservatives, people used fermentation as a way to preserve foods. This seems foreign and odd to many these days, but in considering human history - perhaps we are the odd ones.
Fermented foods are growing in popularity, but I still find more people are unaware of them than those who are aware and consuming them. The more common foods that are fermented are yogurt, sauerkraut, and soy sauce. The lesser known fermented foods are kefir, kimchi, kombucha tea, tempeh and miso. But what are they and why eat them? Basically, fermented foods have had beneficial bacteria added to them. The bacteria begins the breakdown of the food, making it easier to digest and nutrients more available for absorption, and supplying our bodies with good bacteria that supports the immune system.
If interested in trying a new fermented food, kefir would be a good start. It is a nearly lactose free milk product with multiple bacteria strains. I recommend mixing a little with a flavored yogurt. Some suggest pouring it on cereal. If feeling adventurous and want to ferment your own foods, many companies can be found online that supply culture starters for vegetables, yogurts, etc. Regardless of your approach, it is recommended to go slowly at first with consuming fermented foods.