The work of the white men in Africa was to make money with the ivory trade, but also to "civilize" the natives. This term "civilize" means, in the context of the story, to bring civilization.
Marlow was sent to Africa to with the recommendation from his aunt. He was hired to run a steamboat up the river and find Kurtz. However, he "was also one of the Workers, with a capital--you know. Something like an emissary of light, something like a lower sort of apostle." This concept is carried on throughout the story and leads to the great turn of events that happens with Kurtz's character. This term "emissary of light" is repeated in the story to show how they were also going to Africa to bring the light of civilization to an uncivilized people.
This is also shown in the painting that Kurtz had done. The image of "a woman, draped and blindfolded, carrying a lighted torch." "The movement of the woman was stately, and the effect of the torchlight on the face was sinister." This shows the dual nature of this quest. The people went into the darkness bearing the torch of civilization, but as Marlow states, the purpose of the business was to make money. They came in as Gods and tried to spread their beliefs while turning a profit. This is what eventually leads to Kurtz's downfall.