Marlow's contemplative beginning to his story relates his impression of the Romans who may have colonized British shores many hundreds of years before. He explains that even civilized England was once a dark and mysterious place, cold and wet and foreign to the invaders who were tasked with conquering its shores. His reference is to set up his own story, about how "civilized" English citizens such as himself could be changed by trying to fit into or conquer the incomprehensible jungle.
They were conquerors, and for that you want only brute force -- nothing to boast of, when you have it, since your strength is just an accident arising from the weakness of others. They grabbed what they could get for the sake of what was to be got.
(Conrad, Heart of Darkness, gutenberg.org)
Marlow admits that the Romans were not as advanced and purposeful as are modern European expansionists, but their aims are similar; just as the Romans tried to bring their understanding of civilization to English shores (deliberately forcing their own ethos on the natives) so too are the ivory hunters and European expansionists forcing their own ideas on the unwilling natives in the Jungle. This sets up the atmosphere to be sober and even depressing, not adventurous.