What does the author Michelle Alexander mean by "racial caste system" in chapter 1?

Alexander explains how she came to write this book. She was elated with Obama’s election and saw Jim Crow as something of the past. She did not see a new racial caste system, and when she started her job at the ACLU she recognized that there was racial bias in the criminal justice system but that was the extent of it. After she left though, she was convinced it was more than that. The formerly incarcerated are “relegated to a racially segregated and subordinated existence.”