Prebiotics are a group of carbohydrates that provide food for the bacteria that live in our gut. Most carbohydrates, like sugar and starch, are digested and absorbed in your stomach and small intestines. Prebiotics however are not digest and absorbed by our bodies, and pass all the way to the large intestines where most of our gut bacteria live. Here the bacteria can use the prebiotics as a food source. This increases the activity and metabolism of the gut bacteria, causing them to produce more byproducts. Some of these byproducts, like short-chain fatty acids, are thought to be beneficial to human health.
*Prebiotics, a class of carbohydrates, are different from probiotics, which are live bacteria. This study is on prebiotics.