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