Nitric oxide is a gas that carries signals in the body. One cell makes it, it passes through membranes, and it changes what another cell does. Its role in the cardiovascular system, where it relaxes blood vessels, earned Robert Furchgott, Louis Ignarro, and Ferid Murad the 1998 Nobel Prize in Physiology or Medicine (Nobel Assembly at Karolinska Institutet, 1998).
Nitric oxide also binds to cytochrome c oxidase in the mitochondria, where it competes with oxygen and slows the enzyme. One proposed mechanism for photobiomodulation is that absorbed light knocks nitric oxide loose, so that oxygen can return and respiration can resume (de Freitas & Hamblin, 2016).
Where the evidence stands
Nitric oxide signaling is standard physiology. Its release from cytochrome c oxidase by light is a proposed mechanism, part of what researchers call the leading hypothesis for photobiomodulation.
References
de Freitas, L. F., & Hamblin, M. R. (2016). Proposed mechanisms of photobiomodulation or low-level light therapy. IEEE Journal of Selected Topics in Quantum Electronics, 22(3), 348–364. https://doi.org/10.1109/JSTQE.2016.2561201
Nobel Assembly at Karolinska Institutet. (1998, October 12). The Nobel Prize in Physiology or Medicine 1998 [Press release]. NobelPrize.org. https://www.nobelprize.org/prizes/medicine/1998/press-release/