The Charles family stole the piano from the Sutters, the family which had owned them as slaves prior to Emancipation. The piano had originally been obtained by selling two members of the Charles family, and so in appropriating the piano, the Charleses are effectively taking back something paid for with the sweat of their indentured labor. In its own small way, the piano is a kind of reparation for all those years of exploitation and suffering. The fact that the Charleses have their portraits intricately carved into the wood of the piano gives them an added sense that the instrument is somehow a part of them just as they are a part of it.
The Charleses venerate the piano as a historical relic that tells the family story through the years of slavery and emancipation. The significance of stealing the piano is that the Charleses, like African Americans in general, need to appropriate their history for themselves, taking it out of the hands of those who had written it. In that sense, the piano, and the manner of its acquisition by the Charles family, symbolizes African Americans taking possession of their own history and their own experience.