Complex sentence: The term "full disk encryption" is often used to signify that everything on a disk, including the programs that can encrypt bootable operating system partitions, but they must still leave the MBR, and thus part of the disk, unencrypted.
Split into simpler sentences: The term "full disk encryption" is often used to signify that everything on a disk is encrypted, including the programs that can encrypt bootable operating system partitions. But they must still leave the MBR, and thus part of the disk, unencrypted.