Lines 1-5 of the Frost poem is talking about the nature of the wall as natural forces work on it. The affect of the winter forces changes in the ground that alter the nature of the wall. Lines 6-10 addresses the unnatural intrusion of man, who in his quest, will tear down the literal wall in order to more easily capture his prey. Lines 11-25 are still talking about a literal wall, as the two neighbors put the structure back to rights, but it is here where the metaphorical wall is introduced. It would seem natural two talk as the two neighbors go about repairing the wall, but when the one mentions that the pines and apples will never cross the barrier of the wall, the other’s taciturn reply is “Good fences make good neighbors.” Lines 26-30 shows the orchard owner’s reluctance to continue accepting the status quo, and yet rather than voice his thoughts he would like to “put a notion” in the other’s head. The orchard owner then continues ponder this situation by considering what the wall might be keeping out or fencing in. The silliness is highlighted with his exclamation, albeit only in his own mind, of “Elves.” The poem concludes by indicating that nothing will change as the pine tree owner will not go against his upbringing and reiterates “Good fences make good neighbors.”
The following is a link to a great study guide of this poem.