Fences is chock full of baseball imagery, which takes on a lot of symbolic meaning. Troy was robbed of a professional baseball career because of his race. The shadow of this injustice has weighed on him for years and made him a bitter man.
Troy often thinks about life and death in terms of baseball. He describes Death as "a fastball on the outside corner" (1.1.82) and claims he could always hit a homerun off this kind of pitch back in his heyday. Here he uses the idea of baseball to almost taunt Death, daring it to come for him.
Troy also tries to explain his affair with another woman in baseball terms. He tells his wife that when he found her and had Cory he felt like he was "safe" (2.1.116). But after eighteen years of that he saw Alberta and wanted "to steal second" (2.1.118). Rose isn't too impressed with Troy's metaphors and tells him, "We not talking about baseball! We're talking about you going off to lay in bed with another woman" (2.1.121).
There are also actual physical representations of baseball on stage: a baseball bat and rag ball tied to a tree. The fact that the ball is made of rags could be seen as representing Troy's poverty and his tattered dreams. It also shows that after all these years Troy is still trying to hold onto his glory days.
The baseball bat is especially important in the climactic scene between Cory and Troy, becoming a weapon the two threaten each other with. It seems pretty symbolic that Cory and Troy fight each other with a bat, since Troy's inability to play baseball due to racism is what motivated him to sabotage his son's sports career. Now the two do battle with a symbolic representation of this dream deferred.