A little more background info...
Historical Context
Heart of Darkness takes place against the backdrop of some significant historical events. Whilst, you're not required to apply a socio-historical lens to this text, learning more about these events could strengthen your interpretations.
Please click this image to read this very important summary on the Modernist movement within art, literature, and culture; and, more specifically, how Heart of Darkness is a piece of Modernist literature. In fact, many regard HoD as one of the first examples of the Modernist novel.
Colonialism and Imperialism
It is useful to have an understanding of what exactly colonialism and imperialism were. The novel takes place in the Belgium Congo and the Victorian ideology in regards to colonizing and "civilizing" African natives is an important concept to understand when reading. This video by Tom Nicholas, who offers a range of excellent videos on literary and philosophical theories, can help you.
The Scramble for Africa
Another important historical concept to understand is the European "scramble for Africa." As a continent, Africa was sought after by European Powers, attempting to control and exploit its resources. This video offers some comprehensive information about this.
King Leopold II & the Congo Free State (1885-1908)
This video provides comprehensive yet simple information about the Belgium Congo specifically. The Congo river is where the novel is set, and so an understanding as to the treatment of Africans in this area will help with your reading and help you to build empathy.
King Leopold and the Belgium Congo
I kind of love TedEd videos. They're well animated and engaging. This video shines some light on what happened in the Congo under King Leopold in a creative way. Honestly, what happened here is pretty horrific.