Probiotics are live microorganisms that confer health benefits when consumed in adequate amounts and play a vital role in maintaining gut microbiota and overall health. They are naturally present in fermented foods such as yogurt, kefir, kimchi, sauerkraut, miso, and kombucha. Probiotics improve digestion by fermenting dietary fibers to produce short-chain fatty acids (SCFAs), including butyrate, acetate, and propionate, which strengthen the intestinal barrier, reduce inflammation, and support metabolic functions. They also synthesize essential vitamins, inhibit pathogenic microorganisms through antimicrobial compounds, and help restore microbial balance following antibiotic therapy. Growing evidence demonstrates that probiotics enhance immune responses and regulate the gut–brain axis by influencing neurotransmitter production, thereby supporting mental well-being. Certain probiotic strains also contribute to the management of diarrhea, irritable bowel syndrome, hypercholesterolemia, and insulin resistance. Their benefits are strain-specific and depend on adequate viability and clinically validated efficacy. Combined with prebiotics as synbiotics, probiotics provide an effective dietary strategy for promoting gastrointestinal health, immunity, and long-term human well-being.