CSM or Compatibility Support Module is something that allows booting in legacy BIOS mode on UEFI systems.

From Wikipedia:

To ensure backward compatibility, most UEFI firmware implementations on PC-class machines also support booting in legacy BIOS mode from MBR-partitioned disks, through the Compatibility Support Module (CSM) that provides legacy BIOS compatibility. In this scenario, booting is performed in the same way as on legacy BIOS-based systems.

The Compatibility Support Module allows legacy operating systems and some legacy option ROMs that do not support UEFI to still be used.


参考资料:

  1. What is CSM option