Does anyone know how are they taking wavelengths from an image or what exactly "long-wavelength features" means? I googled "wavelength of images" it doesn't se

Does anyone know how are they taking wavelengths from an image or what exactly "long-wavelength features" means? I googled "wavelength of images" it doesn't seem like I am going in the right direction because it's about finding the wavelength of light from images rather than "wavelength of features" that this blog is talking about.