Light given off steadily and without heat enough to burn. Presence made visible in the dark.
Old English glowan (to glow, shine as if red-hot), from Proto-Germanic *gloan; presence made visible in the dark, without the heat that would burn.
A steady light asks nothing of what stands near it, no burn and no cost to closeness, which is why the word is reached for whenever presence should be seen without being felt as heat.