Red light therapy is a form of LLLT, also known as photobiomodulation, which uses rays of a specific wavelength to stimulate hair growth. Although the name includes "laser," LLLT today mostly uses non-coherent light-emitting diodes (LEDs), which can produce light of different wavelengths with specific effects on the human body.
As the name suggests, red light therapy uses red or near-infrared light rays whose wavelength ranges from 630 to 850 nm (660 nm is usually preferred). They penetrate the scalp and optimize cellular function, which in turn stimulates hair growth.