Reactive oxygen species, or ROS, is an umbrella term for reactive forms of oxygen that are a normal part of living with oxygen. At high levels they damage molecules, a state researchers call oxidative distress. At low, physiological levels they work as signals that help cells adapt to change and stress, a state called oxidative eustress (Sies & Jones, 2020).
That double role helps explain the biphasic dose response in photobiomodulation. In lab studies, mitochondrial ROS rose in two separate peaks as the light dose increased, and researchers suggest that ROS acting as a helpful signal at low levels and a harmful agent at high levels may partly explain the pattern (Huang et al., 2011).
As well, broad antioxidant compounds meant to wipe out ROS have not succeeded in clinical trials at preventing or slowing disease (Sies & Jones, 2020). More antioxidant is not automatically better.
References
Huang, Y.-Y., Sharma, S. K., Carroll, J., & Hamblin, M. R. (2011). Biphasic dose response in low level light therapy: An update. Dose-Response, 9(4), 602–618. https://doi.org/10.2203/dose-response.11-009.Hamblin
Sies, H., & Jones, D. P. (2020). Reactive oxygen species (ROS) as pleiotropic physiological signalling agents. Nature Reviews Molecular Cell Biology, 21(7), 363–383. https://doi.org/10.1038/s41580-020-0230-3