The ecological environment in our gut is very important for not only how our gut functions, but how we function overall as human beings, physically and mentally. One of the major drivers for the concentrations of different bacteria in the gut is the type of food that they get. Prebiotics are a type of food (typically fibres) that feed beneficial bacteria in the gut, promoting their growth and activity.
Many of the bacteria in our gut are beneficial and can actually be targeted and have their concentrations augmented by taking probiotic substances, using prebiotics, or even the combination of both.