A luminescent object can be seen even when there are no lights in the scene. It is self-illuminated.
If you are using an image as your texture, load a file to use as a luminance map. The brighter a pixel in the luminance map, the more luminescent the corresponding region of the surface. If you have chosen a Luminance Color and loaded a texture (luminance map) as well, the color will be added at 100% strength to the texture. If you want to see the result without the chosen color, set Mix Strength to 0%.
Luminescent surfaces are used to help simulate objects that seem to be self-illuminated in the real world, such as the windows of an office block at night, neon writing or a TV screen.