The primary symbol in "Mending Wall" is the wall itself. The speaker and his neighbor meet to mend and reconstruct this wall, as they do each year, and they engage in a conversation about the real purpose of the wall. His neighbor believes that the wall, and therefore physical divisions, are necessary to maintain peace. By clearly marking the divisions of their properties, the neighbor believes that they will be able to avoid future conflicts.
The speaker doesn't agree, believing instead that the wall is pointless. The wall divides the speaker's apple orchard from his neighbor's pine trees; the apple trees are not going to cross the property lines, anyway. Yet his neighbor simply repeats an old adage in response to this observation, reminding the speaker that "good fences make good neighbors."
This demonstrates that his need for division is based in tradition and without much thought about the need for those barriers. The wall is symbolic of all the ways humans divide themselves, questioning whether those divisions are successful in maintaining a sense of peace or if the walls themselves represent a sense of cynicism about coexisting with others peacefully.
It is interesting that the speaker grows apple trees. Apples are often symbolic of knowledge, which symbolizes the speaker's sense of wisdom in this conversation. Instead of blindly accepting tradition, he questions whether he and his neighbor benefit from their continual efforts to maintain a division between them.
The pine tree often symbolizes kinship and peace in literature. The neighbor's property is covered in pines. He is focused exclusively on maintaining peace through divisions, believing that his relationship with the speaker depends on clear barriers. He means no ill will toward the speaker and instead focuses on making steady progress through their efforts to reconstruct the deteriorating segments of their wall.
The speaker reflects that this yearly activity is an "out-door game / One on a side." This is symbolic of the outcome of divisions, alluding to the fact that games create both winners and losers.
The symbolism in the poem raises questions about the "walls" in our societies but allows the reader to draw their own conclusions regarding the need for those divisions.