Conrad's use of a frame narrative in Heart of Darkness—a story within a story, as it were—enables the author to distance himself from the horrors of colonial exploitation while at the same time giving him the opportunity to describe them in some detail. In other words, the frame narrative allows Conrad to have his cake and eat it; he can tell the tale while remaining apart from the action in the form of the frame story's unnamed narrator.

This is an especially appropriate narrative technique when one considers that Conrad is dealing with one man's experience of colonial exploitation and how it affects him. As this experience involves a gradual process of disillusionment, it's only right and proper that the story begins not from the point of view of Marlow, but from that of a narrator whose complacency about the values of Western civilization needs to be challenged in no uncertain terms.

In that sense, one could argue that the narrator of the frame story represents the opinion of the average Englishman—someone with an unthinking regard for the imperialist project, whose dark side remains unknown to him. That being the case, it is important to ease the reader into the story, drawing them gradually into the action instead of starting right in the middle of Marlow's steamboat journey up the Congo.

Instead, we start off on the calm, civilized Thames, which is where most of Conrad's readers would've felt right at home. Once they've been lulled into a familiar landscape, they can be drawn into a tale that becomes ever more disturbing as the story progresses.

The narrative form used in Heart of Darkness is the so-called frame narrative of enclosing the main story within an external story. This is common to Gothic works. Other good examples of the practice include Frankenstein (which, in fact, contains a story within a story within a story) and The Turn of the Screw.

There are various reasons why an author might choose to use this sort of narrative technique. Firstly, it was popular at the time this novel was written in and served to characterize the story as part of a particular genre. Moreover, it places an element of distance between the author and the story. The frame narrator who tells the story of Marlowe to the audience aboard his ship is not named; this means that he does not have to take responsibility, on any level, for what he is saying.

Is he an unreliable narrator? How far has he taken liberties with the truth when recounting the central story? What might he have changed or misremembered, and how might this change the truth of what happened? The use of a frame narrative enables us to ask questions about intent and truth and to question how far the story told by our mysterious narrator is true and how far it is simply an expression of his own political or social opinions.

Heart of Darkness is told in first-person point of view as a frame narrative. An unnamed narrator, a sailor like Marlowe conveys to us in written form the oral story that Marlowe tells a group of sailors on a ship on the Thames.

This technique allows the first narrator to characterize Marlowe as a Buddha-like storyteller and a sailor different from (except for liking to tell stories) and more contemplative than other sailors. Likening Marlowe to Buddha at both the beginning and end of the frame narrative lends Marlowe a sense of spiritual authority that he might not otherwise have had and helps us to understand Marlowe as a person who is mystical and set apart.

However, a frame narrative adds another layer to a story: we are hearing someone's account of Marlowe's account, which puts more distance between the reader and the original narrative. There is every possibility that the narrator has—without meaning to—changed, misremembered, or in other ways distorted the story in the retelling, which almost always happens. This lends a more mythic quality to Marlowe's tale.

At the same time, we get the strong sense that the narrator has caught the cadences of Marlowe's voice and thought. He captures the vivid details of Marlowe's sensory descriptions of Africa in a way that put us into the scene and help us to feel the atmosphere of the Congo as if we are there.

Heart of Darkness uses a variety of narrative techniques that add complexity to the story. First, the story of Marlow's voyage into the heart of Africa and his discovery of Kurtz is told as a frame narrative--that is, a story within a story. The story opens aboard the Nellie, a cruising yawl on the Thames, and then Marlow begins to tell his tale about the Congo, which comprises the bulk of the story. 

The other narrative technique in Heart of Darkness is the use of two narrators. While Marlow narrates the story of his discovery of Kurtz and his gradual but sure awareness of the horrors of what the European trading company is doing in Africa, the first narrator is an unnamed sailor about the Nellie. We don't find out who this person is, but it is this sailor who is listening to Marlow tell his tale. The reader only hears this narrator's voice at the beginning of the book and in the very last paragraph, when Marlow stops telling his tale. The only indication at the end of the book that the narrative has returned to the initial narrator is the phrase, "I raised my head." The "I" in this sentence refers to the first, unnamed narrator. 

This "story within a story" technique is called a "frame story". Other examples of frame stories are "The Taming of the Shrew" and "The Princess Bride".

The narrative technique used in "Heart of Darkness", is one of a story within a story.  Although the primary narrator is Marlow,  there is a second narrator, unnamed, who tells us about Marlow telling his story.  There is also a third voice added to this narration which can be considered the author himself, who is really telling the whole story.

"Beyond these three dominant points of view are the individual viewpoints of the book's major characters. Each has a different perspective on Kurtz. These perspectives are often conflicting and are always open to a variety of interpretations."