The setting is the yard which fronts the only entrance to the MAXSON household, an ancient two-story brick house set back off a small alley in a big-city neighborhood. The entrance to the house is gained by two or three steps leading  to a wooden porch badly in need of paint. [...]

The yard is a small dirt yard, partially fenced, except for [in] the last scene, with a wooden sawhorse, a pile of lumber, and other fence building equipment set off to the side. (Wilson, "Setting")

Wilson titled his play Fences because the physical fence around the yard of Troy and Rose's brick house is emblematic of the divides in life that define life for Troy and keep him and his friends out of better opportunities while protectively keeping loved ones in a circle of the known and the watched over. This emblematic fence of Fences also symbolizes the transition and change in Troy's and Cory's lives and in life outside his home fence, good change that Rose, Bono and Cory see but Troy doesn't. There is also a negative significance because Bono, after thirty years of devotion, steps outside Troy's fence physically and metaphorically when he becomes aware that Troy is being unfaithful to Rose.  

This significance relates to the setting of the play (or the "environment" of the play) because all the action and dialogue take place behind the fence that almost encircles Troy's home. The metaphorical message of this is that all of life happens within or outside of fences. The contrary opinion is demonstrated in Rose's insistence that things are different now. She maintains that the fences of life are being brought down while Troy continues to be unable to see this, in fact, he finishes the construction of his home's fence.

ROSE: They got lots of colored boys playing ball now. Baseball and football.
BONO: You right about that, Rose. Times have changed, Troy.[...]
ROSE: They got a lot of colored baseball players now. Jackie Robinson was the first. Folks had to wait for Jackie Robinson.

The significance of the title also relates to the action, which is mostly revealed through dialogue. Troy sums up this relationship to the action when he talks about being "vigilant." Troy is vigilant in guarding life's fences to keep the bad outside the fence and to keep the good inside the fence. The incomplete fence is what let Alberta metaphorically come in to threaten those within. As soon as death takes her, Troy completes the fence to strengthen his battle against death, a physical reality he is not afraid of yet a metaphorical possibility that threatens him and is realized in Cory's and Rose's suffering.

TROY: That's all death is to me. A fastball on the outside corner. ... That's part of life. Everybody gonna die. [...]  I was ready to tangle. Just like I'm ready to tangle now. The Bible says be ever vigilant. ... I got to keep watch.