Probiotics
Live bacteria whose benefits are specific to the strain and dose.
Probiotics are live microorganisms which, taken in adequate amounts, confer a health benefit. The definition is deliberately demanding, and most of the commercial category does not meet it: a product needs a specified strain, a viable dose at the end of shelf life, and evidence for a defined outcome.
The most important principle is that effects are strain specific and condition specific. Lactobacillus rhamnosus GG has evidence for particular kinds of diarrhoea; Saccharomyces boulardii has evidence for antibiotic associated diarrhoea; neither result transfers to a different strain, a different dose or a different problem. A label naming only a genus is telling you almost nothing.
Where the evidence is reasonable is fairly narrow: antibiotic associated and infectious diarrhoea, some IBS symptoms with particular strains, and a few specific clinical situations. Where it is weak is the largest part of the market, which is general purpose probiotics sold to healthy adults for general wellness.
Fermented food is a different and more promising story. A ten week randomised trial found a diet high in fermented foods increased gut microbiome diversity and reduced several inflammatory markers, which is a whole diet result rather than a capsule result.
Feeding your existing microbes is the cheaper lever. Fibre intake shapes the community substantially and has far stronger outcome evidence behind it than any supplement, which is why the boring advice about plants keeps winning.
Safety is generally good in healthy people, with bloating the common complaint, but they are not appropriate for everyone: people who are severely immunocompromised or critically ill should not take them without medical advice.
In practice, if you are buying one, buy a named strain with evidence for your specific reason, and if you have no specific reason, spend the money on vegetables and fermented food instead.