Logs Not Archived - Number of log groups which are not archived, expressed as percentage of total number groups. Please note that the current redo log is always shown as not archived.
You can scroll back in time and see if number of not archived logs was getting close to 100%. If you experience extended periods of time when this statistic is high, you should check/optimize archiving performance.
This statistic is calculated as number of records in v$log where ARCHIVED='NO'.