The central symbolism of Robert Frost's poem, "Mending Wall," is people's tendency to set up barriers, essentially to isolate themselves, despite the futility of doing so. On the surface, the poem tells the story of a farmer and his neighbor mending a wall that divides their property. Yet, in doing so, he comes to realize that, since neither he or his neighbor have animals to contain, the mending of the wall is little more than a ritualized tradition.
Upon realizing the nonsensical nature of the task, the farmer becomes amused, particularly at his neighbor's diligent efforts. He also questions the significance of such an ostensibly purposeless wall, especially in light of the natural environment's apparent determination to break it down.
Even in their attempts to mend the wall, the spirits of the earth seem intent on tearing it down, the pair's labor therefore recalling the Greek myth of Sisyphus. In that myth, Sisyphus was doomed to eternally push a boulder up a hill, only to have it roll back down again upon reaching the top.
Nevertheless, in remarking on the pointlessness of mending one particular wall, the farmer implies that walls in general do have some inherent use or merit, thereby displaying his continued deference to tradition.