Typically this means the cost to repair the vehicle would equal or exceed the car's value, although legal definitions vary.
Vehicles with salt water or hurricane flood damage often have severe corrosion and electrical problems which cannot be properly repaired, so are best avoided.
Damage to automotive unibody frames (commonly used in most cars since 1967 to save weight) requires special expertise and equipment to measure, with factory tolerances typically as tight as 3 mm (1/8").
While some vehicles (such as cars exported overseas after severe collision or damaged before the introduction of mandatory branding) may carry no warnings as to their history, others may be branded as "total loss" when they could have been quite repairable in the hands of someone willing to install used parts and do the work at a more modest price.
These vehicles, if not exported to another jurisdiction with different regulations, will never be registerable or licensed again – even if all needed repairs are made and verified.