The fsck(1M) command has just checked a filesystem, and has determined that the filesystem is clean. The filesystem's superblock, however, still thinks the filesystem is "dirty" in some way.
If you believe that the filesystem is adequately repaired, answer yes to mark the filesystem as clean.
Different "dirty" filesystem types are listed in /usr/include/sys/fs/ufs_fs.h, and include FSACTIVE, FSBAD, FSFIX, FSLOG, and FSSUSPEND.
For more information on superblocks, see the section on checking filesystem integrity in the System Administration Guide, Volume I. If you are using the AnswerBook, "bad superblock" is a good search string.