In this moment of historical reckoning, many Americans are being introduced to concepts like intersectionality, white fragility, and anti-racism. Isabel Wilkerson, the author of the best-selling book “The Warmth of Other Suns,” is introducing a little-discussed concept into our national conversation: caste. As she researched the Jim Crow system in the South, she realized that “every aspect of life was so tightly controlled and scripted and restricted that race was an insufficient term to capture the depth and organized repression that people were living under.” She explains to David Remnick that “the only word that was sufficient was ‘caste.’ ” The United States, Wilkerson argues, is a rigid social hierarchy that depends on a psychological as well as a legal system of enforcement. Her new book is “Caste: The Origins of Our Discontents,” which has already been hailed as a modern classic. She says that “we need a new framework for understanding the divisions and how we got to where we are.”