In short
Photobiomodulation (PBM), also called red light therapy or low-level laser therapy, is the application of specific wavelengths of red (approximately 630–700 nm) or near-infrared (approximately 700–1100 nm) light to skin or tissue at doses too low to produce a significant heating effect.
The leading proposed mechanism is that these wavelengths are absorbed by cytochrome c oxidase, an enzyme complex in the mitochondrial electron transport chain, in a way that can temporarily increase mitochondrial membrane potential and ATP production, and that also releases nitric oxide bound to the enzyme, which can affect local blood flow. This mechanism has reasonably solid in-vitro and animal-model support, though the exact dose-response relationship (wavelength, power density, and duration) that optimizes the effect is not fully standardized across the research.
Human clinical evidence is strongest, and most consistently replicated, for specific dermatological and musculoskeletal applications: several systematic reviews and meta-analyses support PBM for reducing acute pain and inflammation, accelerating some wound healing, and improving skin appearance measures like fine lines and collagen density in small-to-moderate randomized trials. It also has FDA clearance (as a low-risk device, under the 510(k) pathway, not the more rigorous approval pathway required for drugs) for several specific indications including temporary muscle and joint pain relief.
Broader claims made for red light therapy in some longevity marketing — general "anti-aging," systemic mitochondrial rejuvenation, or metabolic benefits from whole-body panels — extend well beyond the more narrowly supported dermatological and musculoskeletal evidence base, and have much thinner or no direct human RCT support at the doses and protocols typically sold to consumers.