Out of all of Frost's richly symbolic poems, this appears to be one of the most controversial in terms of its ambiguity--that is, it is open to opposing interpretations. The poem presents two mind-sets, and it is unclear which of the two is being endorsed. The neighbour expresses the view that walls are useful, while the speaker seems them as barriers that should be torn down. There are also other ambiguities: for example, if the speaker dislikes walls, why does he begin the wall mending every spring?
However, given this ambiguity, and regardless of whether you are for or against walls and their symbolic meaning, I think one of the most interesting phrases in the poem is when the narrator discusses how to think about whether to build a wall or not:
Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offence.
This, to me, lies at the heart of the poem, because when we build metaphorical walls in terms of our identity or who we include, we will always exclude others. It seems to be a truth about wall building that building a wall necessarily entails keeping some in and others out. One the one hand, walls are shown to be necessary, in that they promote privacy, define boundaries and protect rights. On the other hand, walls encourage separateness, isolation, and suspicion. Walls seem to be helpful in some areas and extremely harmful in others.