It means they're below red. Refers to colors that are in the spectrum below that color and correspond to a longer wavelength than visible light in electromagnetic radiation. Infrared rays are invisible to the human eye.
Infrared plural error.
Incorrectly written when going with double r , that is, the correct word is infrared. Infrared or infrared light is electromagnetic radiation of greater wavelength than visible light but less than those of microwaves.