I/O is frozen on database databasename
VSS is being used to backup your database.

Edit: [Thanks Colin]

This causes the logged 'I/O is frozen' message.
It is not serious.
No need to resolve it.

Be warned, your database restore chain may be impacted, in which case it is serious.
If you backup strategy is weekly full, daily diff, hourly tran per day - 8am -5pm; a daily snapshot VSS backup will cause your diff's to fail.