The jungle becomes darker as Marlow travels further and deeper into it. He also travels further and further away from the society which has defined and provided essentially all of his moral and value concepts.
Kurtz "exists" in the heart of darkness, where the moral light of society no longer shines. This is true, not because the tribal people he finds there are immoral, but because he has intentionally chosen to escape and undo his own moral experience in an effort to go beyond it.
There is obvious symbolism in the geography of the jungle then as it relates to morality and society. In the depth of the jungle, where Kurtz waits for Marlow, both morality and society have been left behind.
In an external sense, Marlow's journey into the heart of darkness is a journey up the Congo river, into the "heart" of Africa, the dark continent.
Internally, it is a journey that explores the evil that is in the heart of mankind, summarized so eloquently with "the horror, the horror."
The external significance of Marlow’s journey is that through his eyes the reader sees the evil and injustices as Marlow sees them. Marlow’s journey is an internal one in that his ideas and beliefs are challenged as he must deal with the vileness of the attempts at colonialism or the evil of the renegade Kurtz, and Marlow questions whether personal values such as moral or social choices should have any weight in determining evil.
This could mean one of two things:
Marlow's journey has significance to him as a character/person (internal) and significance to other elements of the story
Marlow's journey has significance in the text, meaning the symbolic value of the journey helps the reader understand the story (internal). Marlow's journey has significance to the real-world events surrounding the text (King Leopold's influcence in the Congo, colonization, etc.)
If your essay deals with the latter, I would highly recommend King Leopold's Ghost by Hochschild as a secondary source.