The frame story has little to do with the Marlow's narrative, but serves to show how different the civilized European world is compared to the brutal jungle life told by Marlow. In the frame story, the imagery and narrative -- technically by a different person, the unnamed narrator -- is warmer and more calm, without the harsh edges and unbalanced philosophizing of the later story. The language shows a world at peace, with people entirely in control of their attitudes and destinies.
The Director, satisfied the anchor had good hold, made his way aft and sat down amongst us. We exchanged a few words lazily. Afterwards there was silence on board the yacht. For some reason or other we did not begin that game of dominoes. We felt meditative, and fit for nothing but placid staring.
(Conrad, Heart of Darkness, gutenberg.org)
The entire scene is set up to cause a mood shift between it and Marlow's story. The unnamed narrator is at ease with the others, and there seems to be no possibility of conflict. This sense lasts just until Marlow begins talking, and the mood changes. Until then, the unnamed narrator gives the scene a sense of camaraderie and friendship, noting that all the people on the boat are "men of the sea" with shared experiences and thoughts. Only Marlow has a different perspective.