Heart of Darkness plays with the genre of quest literature. In a quest, a hero passes through a series of difficult tests to find an object or person of importance, and in the process comes to a realization about the true nature of the world or human soul. Marlow seems to be on just such a quest, making his way past absurd and horrendous "stations" on his way up the Congo to find Kurtz, the shining beacon of European civilization and morality in the midst of the dark jungle and the "flabby rapacious folly" of the other Belgian Company agents.
But Marlow's quest is a failure: Kurtz turns out to be the biggest monster of all. And with that failure Marlow learns that at the heart of everything there lies only darkness. In other words, you can't know other people, and you can't even really know yourself. There is no fundamental truth.