What is a full spectrum LED grow light?
Full spectrum means that a grow light contains every wavelength or color of light a plant needs to photosynthesize, usually focusing especially on the wavelengths the plant is able to most efficiently use and absorb. It covers the range of wavelengths between 390 to 700 nano-moles.
Photosynthesis is, broadly speaking, the process by which plants convert light into fuel; the photochemical reaction by which they feed themselves. Which is why good grow lights are so crucial to plant growth.