During phase 5, fsck(1M) detected that the actual number of free blocks in the filesystem did not match the superblock's free block count. The df(1M) command accesses this free block count when measuring filesystem capacity.
Generally you can answer yes to this question without harming the filesystem.
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.