The Piano Lesson is set in 1937, almost 80 years after slavery was abolished in America. However, the shadow of slavery is still very much felt in the play. The Piano Lesson shows how blacks in America were still struggling to overcome the poverty that slavery left them in. The play also brings to light institutions which created new forms of legal slavery like Parchman Farm, a prison/farm which functioned like a slave plantation. Many blacks were imprisoned there, convicted by a legal system biased against them, and then were forced to work long hours in the fields under grueling conditions. Yes, The Piano Lesson shows that African Americans were still haunted by slavery even though it was no longer legal. This "haunting" is represented pretty literally in the form the ghost of Sutter, a man whose family once owned slaves, and in the form of the piano of the play's title, which is used to summon the spirits of slaves.
When Boy Willie wrestles with Sutter's ghost, it is an allegory for the way the entire African-American race is still struggling with the after-effects of slavery.
The Piano Lesson argues that African Americans can't ever forget their legacy of slavery, because in the knowledge of this painful past lie dignity and strength.