The uncle of the General Manager says this to his nephew within Marlow's earshot. The General Manager and his uncle are rapacious and greedy, and the General Manager has benefited from his apparent immunity to the diseases that befell other Europeans. In the conversation that Marlow overhears, the General Manager and his uncle are speaking about a man in the ivory district who is threatening the supremacy of Kurtz, their most prized agent. The uncle wants this man to be hanged to get him out of the way.

The two men then speak about the General Manager's health, and he says he has been very well, while others are dropping like flies. The uncle tells him to "trust to this," and makes a sweeping gesture to take in the land around them. It's almost as if the uncle is summoning the evils of the world around them and telling his nephew that these nefarious forces are on his side.

This quote was given by the uncle of the station manager while talking to his nephew and pointing towards the forest. Thus,the word "this" refers to the forest. What he is implying is that his nephew should trust in the dangers and climate of the forest to take care of those he fears, including Kurtz and Marlow.