Robert Frost's "Mending Wall" consists of an argument between the narrator about the need for a stone wall that separates their two properties. The neighbor is strongly in favor of the wall; repeats a saying that he heard from his father: "Good fences make good neighbors." The narrator is doubtful about the need for the wall; he says, "Something there is that doesn't love a wall."
The poem reaches an emotional climax as the narrator watches his neighbor repair the wall after it has suffered damage over the winter season:
I see him there
Bringing a stone grasped firmly by the top
In each hand, like an old-stone savage armed.
The narrator seems to feel that the neighbor insists on preserving the wall as a way of preserving emotional distance between himself and the narrator. The neighbor is territorial, like a primitive human or a beast. The neighbor "moves in darkness." It is not merely a physical darkness created by "the shade of trees," but a emotional-psychological darkness in which he refuse to connect to other people.
I think the narrator is referring, in part, to the darkness of the past, the darkness of maintaining a tradition that no longer seems relevant. The neighbor who insists on keeping the wall claims that "'Good fences make good neighbors,'" but the narrator would like to "put a notion in his head": he wants to question why good fences make good neighbors. There are no cows to prevent from wandering onto a neighbor's property, so what is it exactly that the wall is meant to keep in or keep out?
The narrator wants to question the tradition of maintaining the wall, despite the fact that he works to maintain it as well, because each man only grows trees, and the trees will not bother one another. The origins of the wall seem to have been forgotten; these neighbors just keep on maintaining it, though the one really has no idea why. The wall's origins are dark; the reasons to maintain it are dark -- no light illuminates the reasoning used by the neighbor who so wants to maintain the wall. His ideas seem antiquated, alienating, and without logic.