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