The central image is the "heart of darkness" itself, which could be either the inner part of a man which can turn to evil, or the jungle's deep interior where men forget their civilization and become primal beings. The phrase occurs regularly, although not always in that exact order, and Marlow is powerfully affected by the idea that the instinctual, savage side of humanity could be released simply by venturing into the jungle. He also uses the phrase to describe Kurtz, specifically speaking about Kurtz's deepening insanity:
"Kurtz discoursed. A voice! a voice! It rang deep to the very last. It survived his strength to hide in the magnificent folds of eloquence the barren darkness of his heart."
(Conrad, Heart of Darkness, gutenberg.org)
The "heart" itself may be the metaphorical true self of man, the secret and even unconscious side which is hidden until pulled out by extraordinary circumstances. It could also be the deeper part of the jungle where there is no "proper civilization," and where people live according to their instincts, not artificially-created morality. In either case, the idea of a hidden place where "normal" behavior becomes alien is central to the themes of the novel.