This is a great question, Keira. The literary device that Conrad uses is called a "frame narrative," which is a fancy way of describing a story within a story.
Try thinking about the concept of the narrator in any book that you are reading. Often, we understand that we are the audience when we read a story. But what if we are not the audience? What we are reading in Heart of Darkness is "filtered" through the story-telling device. That means that there are two audiences, the audience in the story and the general audience that reads the book.
Audience is very important in writing because a story is tailored to meet the needs of the audience. You are likely to tell a story a different way to different groups of people, for example, to your parents or your friends. You might have a different tone or different word choices. You might select different details. You might want to make different points. You will want to impress different audiences in different ways.
Consider the narrator's audience in this story. Who are these men? What is the setting? What points does the narrator want to make to his audience? Are they different from the points the author wants to make to you? What details are important to this audience? What kind of tone might be important? Is he trying to impress this group of men? What impression does he want the group to have of him? Is his tone different than if there were a general audience?
When there is a story within a story like this, does it make us feel somewhat "removed" from the story? Can we be more objective about the story because it is not being told to us?
These are the kinds of questions that you need to think about to answer this question. Try taking a few passages or an incident from the narration and use these questions to analyze the different effect that is created with this literary device.
Good luck to you. Lorraine