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