The fsck(1M) command is checking the file system shown in the messages that are displayed before this one. The first phase checks the inode list, finds bad or duplicate blocks, and verifies the inode size and format.
If more than a dozen errors occur during this important phase, you might want to restore the file system from backup tapes. Otherwise, it is fine to proceed with fsck(1M).
For more information, see the chapter on checking file system integrity in the System Administration Guide, Volume 1.