Simply speaking, prebiotics is a food for Probiotics bacteria. So, for the good bacteria to function well, they need to consume food too.

Common prebiotics include dietary fibers such as inulin, fructo-oligosaccharides (FOS), and galacto-oligosaccharides (GOS) found in foods like garlic, onions, bananas, asparagus, and whole grains. A combination of both is often recommended for optimal gut health, and this synergistic relationship is sometimes referred to as a "synbiotic" approach.

In a more scientific definition, prebiotics is defined as a group of nutrients that are degraded by but microbiota. Their degradation products are short-chain fatty acids that are released into blood circulation, consequently, affecting not only the gastrointestinal tracts but also other distant organs. Fructo-oligosaccharides and galacto-oligosaccharides are the two important groups of prebiotics with beneficial effects on human health. (source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6463098/)