Glow is a rendering term which means to define a material which appears to be illuminated by lights, even if it is in a shadow, or the lights are dim, as in a night scene.
Uses of Glow
Glow can be applied to windows which are illuminated from the inside of a building, neon lights, or other lights, such as a car tail light, which would look Photorealistic if they were self-illuminating, but do not need to cast light on nearby objects.
Sometimes a headlight or a taillight needs to have a lamp in it, so that it can illuminate the road, or other objects, but sometimes it is sufficient to apply a glow material instead.
Glow vs Using Lights
To be completely realistic, the user should place light sources inside each segment of the neon tubing, or inside every light of a car, and then use transparent materials for the tubing itself or the lens of the light, but the is sometimes excessive and unnecessary for most renderings.
Making an image glow
You may have an object in your model which is backlit, or produces its own illumination, such as a television set. You can make it glow brighter or look more realistic by adding self-glow from the material editor.
Note: Items with self-glow will seem brighter, but will not illuminate other items in the room. If you wanted the television to illuminate a dark room, you would set it to be a light instead.