The concern flag on volunteer notes is similar to the safeguarding flags on family notes, other than it's entirely internal.
If you have a concern regarding a volunteer, for instance around adding notes in a timely manner, then you should flag this as a concern when leaving a note on a volunteer record.
This will be notified to either the CVM team leader or the Senior CVM, as appropriate. Once a first concern has been raised, you'll see the options change. You can now mark the concern as 'cleared' or mark that it's been raised a second time. Once a second raising of a concern is in there, the options change again, either to clear the concern(s) or to pause volunteering.
At any point, when you clear the concern(s) by adding a new note with 'clear current concern', the system resets for that volunteer and the new options are either 'no new concern' (the default) or 'first concern'.