Boot Configuration Data (BCD) is a critical data store used by the Windows operating system to manage boot settings. It contains information about the operating system loader and the various operating systems installed on a computer. BCD replaces the older boot.ini file used in previous versions of Windows. The BCD allows users to configure boot options, such as selecting which operating system to start or enabling debugging features. It is essential for troubleshooting boot issues and can be modified using tools like bcdedit, which provides a command-line interface for managing boot configurations.