On every system boot up, RedHat Linux normally checks the integrity of all the required filesystems. It fixes the filesystem errors if there is any. The purpose is to ensure that a booted up system can access a proper filesystem. Further usage of a broken filesystem may devastate the filesystem which makes recovery of precious data impossible.
The essential utility for checking and fixing filesystem is fsck. You seldom use this command at this stage. Keep this command in mind, you will need it in the future. For detail, see fsck manpage.