This quote comes from the final section of the novel, when Marlow goes to visit the Intended and shared with her the final words of Kurtz and to pay his respects. It is vital that you realise the symbolism of the darkness and the light in this quote and indeed, in the section as a whole, as the way that the light coalesces around the Intended's forehead is repeated more than once. Analysing how this image is used in the text helps us to reveal its meaning. Consider, for example, how the forehead is described later on. In response to the Intended's claim that she knew Kurtz best, note how Marlow responds and how he describes what he sees:
"You knew him best," I repeated. And perhaps she did. But with every word spoken the room was growing darker, and only her forehead, smooth and white, remained illumined by the unextinguishable light of belief and love.
If we consider the conversation between the Intended and Marlow, we see that the Intended does all the talking, and Marlow basically agrees with her, keeping silent. The Intended expresses an unshakeable belief in Kurtz and his ability and what he achieved, which we can relate to the ignorance of so many people surrounding the colonial endeavour. The fact that it is the forehead that is white, which is where the brain is, emphasises the way in which our brains and thinking can become seduced by big narratives such as the colonial story of saving and helping those less fortunate than ourselves. However, Marlow, after his experiences, can only note the darkness that surrounds the Intended and the way that her forehead and her words points towards her complete ignorance about both Kurtz and the true nature of colonialism, and of man himself.