In the case of this play, the title indicates the protagonist's struggle. Over the course of the play, Troy is challenged to exceed, supersede or defeat his limitations in a number of ways. The fences of the title can be understood to represent these limitations.
He is challenged by the prospect of death and tells stories that hint at Troy's need to change his own nature and his inevitable inability to face reality. We see this as Troy assures himself and others that he has faced death and defeated death in a wrestling match.
Later, Troy depicts death as a baseball pitcher. In Troy's vision, he is the batter and he has a real chance to knock the ball out of the park, over the metaphorical fence, and in this way take power of nature.
Further examples of the limitations that Troy faces can be seen in his emotional struggles in the play.
Unable to open up to those that he loves, Troy keeps much of his emotion inside, building imaginary fences between himself and his family and friends.
Troy has also experienced practical limitations in his life relating, being imprisoned for fifteen years, being kept out of major league baseball, and being unable (for a time) to attain a promotion at work. Some of these limitations are real and race related and some are matters of perception, yet these are equally real for Troy.
The fences in the title also have a resonance, both literal and metaphorical, with Troy's baseball career. Troy is an "all or nothing" person, largely incapable of compromise. He cannot soften himself and cannot yield to those he cares for. The mentality implied here is the same as that which characterized his style of play in baseball.
His successes are hits over the fence, but his failures are strike-outs.