The narrator and his neighbor have a disagreement about the importance of mending the wall between their properties.
The narrator thinks that mending the wall every year is silly. They have no livestock to keep in, and his apples don't threaten his neighbor's pine forest. He also feels like something doesn't like the wall, maybe like something in the world is taking it down on purpose, and that's why, when their backs are turned, new cracks appear and the stacked rocks fall back out of place.
The neighbor, on the other hand, stubbornly repeats his father's wisdom: "good fences make good neighbors." He is content to make the yearly trip along the property line in order to ensure that the wall can continue to exist, even if it will always come back down.