On ct scan what does hypodense regions noted in the liver parenchyma be consistent with unopacified vascular structure mean?

It means that area is likely a vessel in the liver (vein or artery) but the scan was done without IV contrast so the content, i.e. blood, inside the vessel could not be confirmed (unopacified). If contrast is in blood, the image on scan will become white and it is called opacified. You will likely need a CT scan with IV contrast to confirm that finding is truly a vascular structure.