For the narrator, tradition by itself is unimportant. In his case, it creates unnecessary work that serves no good purpose. For his neighbor, tradition is all-important and must be adhered to for tradition's sake.
The tradition in question is the annual mending of the stone wall between the two men's properties. Every year, the two meet for the laborious process of repairing gaps in the wall.
The narrator thinks it is ridiculous to worry about mending the wall, as neither of them keeps livestock that could damage the other one's crops. The speaker grows apple trees and the neighbor pines: they don't need a wall.
The neighbor, however, learned from his father the saying that "good fences make good neighbors" and doesn't question the truth or logic of that statement. The narrator does, asking himself, "why do they [fences] make good neighbors?"
(The entire section contains 2 answers and 421 words.)