jmp242,
This will vary by drive manufacturer as each manufacturer can implement SMART thresholds as they see fit PC-Doctor reports what is sees in the SMART logs. Generally this indicates one or more of the SMART values are approaching their preset limits. When they reach or exceed these limits it will change from a pre failure warning to an imminent failure warning.
There have been instances where this failure was a false positive so I would search the drive manufacturer for the model you have and see if this is the case. If it is not and the system contains critical data backups are a must and the drive should be scheduled for replacement in the near future.
-Fred
