What does Marlow mean when he states "idea that redeems the ugliness of the conquest of the earth? His statement can be found on page 9 of the book.
Marlow is talking about "civilizing" the Congo. He believes that by going he is an emissary of light to the natives. Since Marlow has yet to go to the Congo, his beliefs reflect the belief of the company. This is important because it shows that while the pretense is civilization, the reality is greed.