If we look at the first section in this incredibly powerful novella, Marlow recounts to us how as a child, he had a fascination for maps. He talks about how he would spend hours looking at them and thinking of places he would like to go. One of them was the North Pole, but he says he will not go there now, because "the glamour's off." Obviously, now that he is an adult, the childlike enthusiasm and innocence that made him desire to go there has abated.
However, what attracted him to Africa was the way that it was "the biggest, the most blank" out of the continents. However, as Marlow grows up, so Africa becomes more explored and charted, and rivers and other geographical forms are recorded on maps. It was no longer a place of "delightful mystery" and had instead become a place of "darkness." However, there is one river in particular that seems to draw Marlow and tempt him:
But there was in it one river especially, a mighty big river, that you could see on the map, resembling an immense snake uncoiled, with its head in the sea, its body at rest curving afar over a vast country, and the tail lost in the depths of the land. And as I looked at the map of it in a shop-window, it fascinated me as a snake would a bird--a silly little bird.
Note how the river is compared to a snake--a symbol of temptation and evil, and it is said to "charm" Marlow as if he were a bird. It is this that makes Marlow decide to go to Africa and experience the "darkness" of this place for himself.