Beth Newman writes:
The unnamed narrator of Conrad's Heart of Darkness offers a well-known cautionary metaphor for reading narratives-particularly frame narratives, fictions like Frankenstein, Wuthering Heights and The Turn of the Screw (as well as Heart of Darkness itself) that contain a story within a story. The narrator tells us that for Marlow the meaning of a story "was not inside like a kernel but outside, enveloping the tale which brought it out only as a glow brings out a haze." Conrad does not define precisely what "inside" and "outside" are, which leaves considerable leeway for the reader. But he does extend the metaphor, restating it by figuring it in the formal structure of the text. He offers us Marlow's story not by itself, but embedded in another-enclosed in a frame that completes it and is completed by it, "as a glow brings out a haze." By tracing the narrator's conversion in the frame from a complacent faith in the superiority of the West to Marlow's anti-imperialism, Heart of Darkness makes meaning something that happens on the margins, along the edges of a narrative, as much as something we discover within it. We cannot hope to extract a single "kernel" of meaning from Marlow's story, nor from those of Frankenstein, Nelly Dean, or James's governess, but must attend instead to the relations between the stories in the center and those in the frame, and listen to the dialogue between the voices that speak them.
Quoted from Beth Newman, "Narratives of Seduction and the Seductions of Narrative: The Frame Structure of Frankenstein ELH (1986): 141.