A postcolonial view is one that critiques colonialism, usually, but not always, from the point of view of the colonized.
Conrad sets up his postcolonial critique in the frame story. Before Marlow gets to the tale of Kurtz and Africa, he sits on the deck of a ship on the river Thames, near the heart of Britain's empire. Here, he ruminates about the Roman conquest of Britain thousands of years earlier. He imagines that Romans came to a backwater like Britain with mixed motives and that the Romans saw the British as as savage as the British today find the Africans. Marlow states of the Romans,
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 .... The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much.
The Roman conquest of Britain, Marlowe implies, is much like Britain's current conquest of weaker nations: it is a matter of brute force, and there is nothing redeeming about it. This establishes from the start a theme that Marlow will return to, that the noble and selfless reasons articulated in the popular press to justify incursions into places like Africa are a pretense. In fact, European imperialism is evil.
The postcolonial theme is reiterated in the response of Marlow's aunt to his being hired as a steamboat captain in the Congo. She sees Marlowe as participating in a good and idealistic venture that will help civilize the continent. This leads Marlow to think, first, that she is repeating the "rot" that has appeared in "print" about the nobility of imperialist ventures and then to question if she understands that the the Europeans are in Africa for "profit."
Thus, from early on, the novel establishes that it will look at colonialist and imperialist adventures with more realistic, critical eyes.