Infrared, like ultraviolet at the other end, is invisible to the human eye: it is electromagnetic radiation of longer wavelength than red, discovered by William Herschel in 1800 when he measured a rise in temperature just beyond the red edge of a spectrum cast by a prism and realised there were rays there that carried heat but no light. Everything warm radiates in the infrared, which is why thermal cameras can see in the dark.
In modern esoteric colour systems infrared serves as the band below the root, the invisible red-ward extension of the chakra rainbow, associated with the densest, most physical and instinctual level, the body's survival heat beneath the visible spectrum of the subtle centres. As with ultraviolet above the crown, this is a twentieth-century analogical use rather than any traditional attribution; the older chakra sources know nothing of infrared, and the extension of the visible rainbow into the invisible bands is a recent feature of aura and energy literature. Physically, infrared is simply radiant heat.