When a high priority operation attempted to acquire a specific volume, it was being used. To free the volume for this operation, the client session identified is being cancelled by the system.
The lower priority client session is cancelled to free up the volume.
Restart the session that was cancelled, it will wait until the higher priority operation if finished with the volume. This cancelled session was most likely a backup/archive session and you may just want to let it restart automatically during its next scheduled backup window.