The gastrointestinal tract has the largest surface area of the body and if it is subjected to continuous stress, it will activate the immune system. The intestinal barrier, the immune system, and the microbiota together form a line of defense against infection in order to maintain a healthy balance between the host and the microbiota. Nutritional strategies such as feed additives can be applied to maintain homeostasis in the gastrointestinal tract. When this equilibrium is disrupted, metabolic dysfunctions can occur. Postbiotics—the metabolic and/or structural compounds of microorganisms—can stimulate anti-inflammatory, antioxidant, immunomodulatory, and enzymatic effects, providing a safe alternative to establishing intestinal eubiosis. In addition, postbiotics can be used as an alternative to antibiotics in broiler diets by maintaining productive performance under challenging conditions. This review provides an overview of the postbiotic concept, mechanism of action and production, the evidence for the action in the gastrointestinal tract, and applications in poultry production.