These companies are fighting for a cut of the billions upon billions people spend annually in pursuit of gut peace. But what, exactly, are prebiotics and probiotics in the first place? Put another way, they’re food for the “good” bacteria that call your gut home. Among other roles, gut bacteria help you digest food, interact with your immune system, and may even influence your mood. Basically, there is some evidence that probiotics can help prevent and treat diarrhea, and they may also help relieve constipation.