Probiotics are live microorganisms that, when consumed in adequate amounts, provide a health benefit to the host — in particular by supporting the balance of the gut microbiota.
Not all strains are equal: Lactobacillus and Bifidobacterium are among the most studied, but effectiveness depends on the specific strain, dosage, and the condition being addressed (for example irritable bowel syndrome, recovery after a course of antibiotics, or general digestive wellbeing).
Good food sources include yogurt, kefir, sauerkraut and other fermented foods. A supplement can be useful in specific situations, but should be chosen — ideally with guidance — based on the strain and the reason it’s needed, not picked at random off a pharmacy shelf.
This is a short starter draft — it will be expanded with more detail and supporting evidence.