What is ' Our Lady in chains"?

At Lily’s first meeting of the Daughters, August retells the story of Our Lady of Chains. The story involves a slave, Obadiah, finding a wooden figure of Mary. Obadiah retrieves the statue, and he hears the statue reassure him that she will take care of him. He brings the statue to the prayer house, and another slave named Pearl explains that the statue is the mother of Jesus. The people celebrate the statue and find strength from touching her heart. Soon, the master discovers the statue and chains it to a barn. Soon, however, the statue is found to have broken the chains.