The title of the novel suggests major themes developed through Marlow's journey toward Kurtz and his ultimate confrontation with him. In the novel, Marlow actually makes three journeys, each one taking him into the heart of a different kind of darkness.
He makes a journey into the heart of Belgian colonialism, into ivory country where he observes the horrendous effects of colonial policy. Marlow knows intellectually that trading companies are run for profit, but he is not prepared for what he finds in Africa. He is filled with disbelief and moral revulsion by the corruption, greed, exploitation, and human suffering he witnesses.
Symbolically, Marlow's journey suggests a mythic journey into the underworld as he moves through several "circles of hell." He moves from Brussels to the African coast and then through several trading company stations until he arrives at the inner station, "the bottom of there." It is here he finally meets Kurtz, the devil incarnate. Kurtz, Marlow observes, lives in "impenetrable darkness."
A final journey into darkness that Marlow takes is his journey into the unconscious self. At the beginning, Marlow says he does not know himself; his trip into Africa becomes a journey toward inner truth. In Kurtz, Marlow meets the Freudian id, his brutality and self-indulgence unchecked by conscience or social restraints. In dealing with Kurtz, Marlow faces his own capacity for evil, the darkness within himself, but he turns away from it. Marlow has "peeped over the edge" into the darkness of inhumanity, but he escapes, learning wisdom at price: the loss of innocence about himself and the human condition.
Marlow's journeys into the heart of darkness change him forever. When he returns to Brussels, he lies to Kurtz's fiancee about Kurtz's character and activities in Africa. He makes a moral choice, choosing the lie over the truth, because he embraces human charity.
The title of the novel, Heart of Darkness, by Joseph Conrad, has symbolic significance to the story, and it is a literal reference as well.
Literally there is reference to the color of the jungle's foliage:
'The edge of a colossal jungle, so dark-green as to be almost black, fringed with white surf, ran straight, like a ruled line...'
"Heart of darkness" can also refer to the evil that exists within the white overseers at the first (Lower) station. They guard black workers and treat them intolerably as they men carry baskets of dirt on their head. Marlow, our main character, likens what he sees to hell. The workers are weak and starving, chained around the neck and to each other, and they walk with "deathlike indifference." They are beaten at the discretion of their guards. Other things going on around them seem to indicate if not madness, then a lack of reason: dynamiting a cliff that needs not be destroyed, and old wreckage rotting on the landscape. This is his first glimpse of the "heart of darkness" that rests within the frame of men.
"Heart of darkness" refers to the the unknown territory in which Marlow travels. It is a place of mystery and the fear—as Marlow travels to a place totally unfamiliar, where death lurks around the corner and unknown assailants in the jungle fire on his ship.
Finally, when Marlow meets Kurtz, we see the true "heart of darkness." Kurtz has become the essence of one with a blackness in his soul. First, the man has become a god among the natives.
'The camps of these people surrounded the place, and the chiefs came every day to see him. They would crawl...'
Regarding Kurtz's "psychotic break:"
'Mr. Kurtz lacked restraint in the gratification of his various lusts...But the wilderness had found him out early, and had taken on him a terrible vengeance for the fantastic invasion. I think it had whispered to him things about himself which he did not know...and the whisper had proved irresistibly fascinating. It echoed loudly within him because he was hollow at the core...'
Whatever frailty in Kurtz's character or psyche, Marlow believes the lack of civilization with the jungle and the disregard for life, turns Kurtz, once seen as a brilliant member of the company, mad. Even as they leave the Inner Station on the ship, Marlow provides a haunting, compelling image of Kurtz:
He was an impenetrable darkness. I looked at him as you peer down at a man who is lying at the bottom of a precipice where the sun never shines.
eNotes's character description of Kurtz summarizes how we can apply the term "heart of darkness" to Kurtz:
Kurtz, previously known to Marlow by reputation...is revealed upon acquaintance to be a dying, deranged, and power-mad subjugator of the African natives. Human sacrifices have been made to him. Rows of impaled human heads line the path to the door of his cabin. Kurtz is both childish and fiendish...His brain is haunted by shadowy images.
"Heart of darkness" could refer to the inner-recesses of the jungle, to the evil in men's hearts, to the fear of the unknown in this territory where the rules of men do not reach, and ultimately to Kurtz's loss of sanity in the heart of the Congo.