The setting of Conrad's Heart of Darkness is extremely important to the story. The Congo region, to which Marlow is traveling in order to find and return with Kurtz, is a relatively unknown portion of the world.
European explorers first discovered the Congo River in 1482 and maintained a presence on it for hundreds of years thereafter, never traveling more than two hundred miles upstream.
Explorer Henry Morgan Stanley was finally able to chart the course of the Congo River and all its tributaries (except one) to the Atlantic Ocean. This provided much easier access to the interior of Africa, where ivory and eventually rubber, valuable resources of the African continent, could be harvested and shipped all around the world. Leopold II of Belgium "claimed" the area and began to export goods that were in such high demand. However, to do so, the native population was enslaved and victimized. Hands and feet were chopped off with regularity, and vast numbers of people were murdered and dumped into rivers.
It is into this horrific situation that Marlow travels. Kurtz works for the Company, as does Marlow, and has been responsible for collecting and exporting more ivory than anyone else. He has not been heard from for a long time, so Marlow is sent by steamboat (and by land) to bring him home.
The "darkness" referred to in the title could easily be thought to describe the heart of the jungle. But very soon after he arrives on the continent of Africa, Marlow is horrified by the conditions: the waste of resources, time, and most especially human life. In one place Marlow sees machinery abandoned and rusting. On a nearby cliff, explosives are being detonated with no purpose, for they are not being used to make a road or clear the land. Men are enslaved:
A slight clinking behind me made me turn my head. Six black men advanced in a file, toiling up the path...the clink kept time with the footsteps...I could see every rib, the joints of their limbs were like knots in a rope; each had an iron collar on his neck, and all were connected together with a chain...
These men are chained together and forced to work at gunpoint; they have given up hope. They are treated like criminals in their own land.
In other places Marlow sees men who can no longer move: they are slowly dying. He feels greatly out of place, disgusted by these things. It is through his eyes, then, that we can experience the terrible situation the Europeans have created to strip the land for personal profit. The darkness no longer applies only to the shadowy jungle, but to the blackness of men's souls.
This is a land of mystery, and what is unknown is used to create the mood and influence the reader. With every atrocity Marlow witnesses, the reader is more appalled; we learn more about Kurtz and Marlow—the setting provides the opportunity for characterization. Part of the suspense of the story rests in the unknown dangers in the jungles. This also comes from the story's setting.
All this prepares the reader for butchery, human sacrifice, and Kurtz's complete moral degradation—the same man who is worshiped by the natives as their chief...like a god. When Kurtz is found, he is completely mad and physically ill—changed by what he has done and what he has seen.
The setting affects the mood, the characterization and the plot development. The setting allows for more realistic plot development, and as a result, more credible characters.