Boot in single user mode (hold down cmd+s while booting).
In single user mode, write
"fsck -fy"
Run the command, and when it's done it should say that it's made changes
Then you can write "exit" to boot your Mac
It just means that there are a few issues with the file system. Likely nothing serious. HFS+ gets corrupted from time to time. Metadata not being written properly or something like that. It happens