Heart of Darkness was set in the Belgian Congo. There are a number of ways one can decode the title. Simplistically, Congo is at the heart of Africa - known as the dark continent by authors & the like around 100 years ago. Obviously, the dark continent is linked to the language of colonial exploitation & it is considered bad form to refer to Africa as 'dark' nowadays. The dark heart of Kurtz whose methods & practices have become 'unsound' (Coppola), due to his prolonged exposure to Africa (Europeans widely thought that 'madness' could result if white men who operated in the colonies spent a long time outside the familiar constraints of their own societies, giving rise to concerns that mad Europeans could threaten their image of superiority - read Sadowsky, McCulloch, Vaughan etc.), again, fairly simplistic, but easy to back up. A more radical approach would be to explore the dark hearts of those who tried to legitimise colonialism by claiming that Africans were inferior to Europeans in terms of intellect - the travel writers, anthropologists & ethno-psychiatrists such as Virey, Cuvier, De Blainville & later Carothers. These guys facilitated, rationalised & justified colonialism in the name of pseudo-science concurrently with missionaries in the name of religion. Conrad has been accused of facilitating notions of the inferiority & savagery of Africans (Achebe), although Hearts of Darkness is generally viewed as an indictment of colonialism. Read Sartre's preface in Fanon's The Wretched of the Earth, however, be warned, this is controversial stuff & may not be suitable for 12th grade. Probably the safest thing to do is read some scholarly reviews on H of D & remember that darkness can also be seen as not illuminated as in the dark ages where knowledge of what happened through that period was scant.

Titles of novels mostly relate to their main themes or motifs. In Conrad's novel, Heart of Darkness, the theme is the discovery of darkness, not only physical, more pronouncedly moral and spiritual. The word 'heart' signifies the innermost core, and the word 'darkness' refers to the moral-spiritual vacuum that the ivory trader Kutz got himself trapped in.

Conrad weaves a complex narrative in which Marlow tells the story of his visit to the deep dark heart of Congo to see and rescue Kutz from the 'Inner Station'. When after a long and arduous journey from the 'Central Station',  Marlow reaches the darkest part of Congo, Kutz is found among the black Africans who attack Marlow's vessel, for they wouldn't allow their god-like Kutz to be taken away. Infirm, indisposed and crawling through the forests, Kutz is rescued. But he succumbs on the way back, uttering the words-'horror! horror!' Perhaps, Kutz could see into the deep darkness of his own mind.