What are the symbolic meanings of fences?How effective is this choice of symbol?

Are you perhaps referring to Robert Frost's poem 'Mending Wall' here? Of course he is not the only writer to discuss the themes of barriers, divisions, fences and walls in our society and in Nature.

In that particular poem, Robert Frost actually talks about two forms of human-made barier fences and walls. He begins by challenging the old saying 'good fences make good neighbors' in which man protects himself from confrontation and loss of privacy as well as damage and loss of property through the building of sound fences. The better the fence, the better (more considerate,dependable,respectful) the neighbor.Frost challenges the assumption that all things think the same about the benefits of this arrangement:

'Something there is that does not like a wall'

Then Robert Frost goes on to tell us what it is. It could be nature (such as the soil, water, moles, birds) which totally ignore man's superficial and pointless barrier and run under, across or on top of it. Some people think that he may have been referring to the Cold War or even the Berlin Wall here. Frost was actually in England at the time he was inspired to write this poem...he was reminiscing about his much-beloved walled landscape back home in the US.