What does marlow mean when he says ,"takes him away from the truth of things"?

His journey down the African coast takes him into a completely different world, a world in which his usual reality has changed. The truth that he knows is no longer relevant to what is going to happen. He goes into a world with which he is unfamiliar and which will treat him in ways he is unfamiliar with. He has to learn to think differently when he encounters the challenges in the African world.