The "heart of darkness" has both a specific and a universal meaning in the novella. It refers particularly to the horror, cruelty, and greed that Marlow experiences in the Congo. In this iteration, it focuses on Kurtz, who crystallizes and symbolizes all the horrors of the European actions in Africa.
Marlow notes that as the boat he captains on the Congo gets closer to Kurtz, it gets closer to the heart of darkness:
[we] crawled towards Kurtz ... We penetrated deeper and deeper into the heart of darkness.
The darkness is, on one level, the oppressive geography of Africa: its dark jungles, its miasmas, its claustrophobia, its fogs. It is a landscape of horror as seen through Marlow's eyes. But the true heart of this darkness is the penetration into this continent of people like Kurtz, whose pursuit of profit at all costs and setting themselves up as godlike figures exemplifies all that is wrong with European imperialism. Kurtz's soul sickness is Europe's.
The frame story that surrounds Marlow's tale of his time in Africa underscores that this heart of darkness is more, however, than simply the singular experience Marlow had in Africa. The cruelties of European imperialism, of conquest, of the strong suppressing the weak, are universal. As Marlow states, what the English do in Africa or around the globe now is no different from what the Romans did to the ancient British. Humanity has a heart of darkness. The novel ends with this image of the sea route from the Thames to the Atlantic:
The offing was barred by a black bank of clouds, and the tranquil waterway leading to the uttermost ends of the earth flowed sombre under an overcast sky—seemed to lead into the heart of an immense darkness.