Protein fragments (usually from wool or feathers) that coat and fill damaged hair so it feels smoother and breaks less. A cosmetic effect that washes out.
Hair is itself made of keratin. Products add keratin broken into smaller pieces ('hydrolysed'), which stick to the hair surface and, if small enough, slip a little way into damaged cuticle. Hair feels smoother, looks shinier and may break less while the coating lasts.
This is different from salon 'keratin smoothing' or 'Brazilian' treatments, whose straightening effect usually comes from formaldehyde-releasing chemicals heated into the hair, not from keratin.
Laboratory tests, mostly by ingredient makers, show hydrolysed keratin binds to hair and improves strength, gloss and combing. There are no independent trials on long-term hair health, and nothing shows it rebuilds hair structure or affects growth.