Apple's Photos app frames image noise as a consequence of how much light-information a region of the photo carries, not just a generic 'grain' artifact: brighter areas capture more light information and therefore show less noise, while shadows and underexposed areas carry less information and show more grain ('Noise shows up where we have less information'). This reframes the Noise Reduction slider as an information-density fix rather than a blanket smoothing tool.
Noise Reduction removes grain but softens the image as a side effect — pushed too far it strips out real sharpness and detail along with the noise. Recommended starting point: ~40, not pushed to 100.
Because Noise Reduction softens the image, the recommended workflow is to apply it first, then add back a little Sharpness afterward to counteract the softening — the two sliders are treated as an opposing, matched pair applied in the same editing pass.
Noise is 'very fine granular detail' that isn't visible at full-screen/social-media viewing sizes — the presenter frames fixing it as mattering mainly for printed images, and skippable for photos posted to social media.
See Mono Filter + Auto Tool as iPhone Photos App Editing Baseline for where this slider sits in the overall editing order, and Vignette as Subliminal (Should-Be-Invisible) Eye Guidance for the companion Vignette slider.