The original miles on a vehicle means that the vehicle with the miles has the same original engine, original transmission and some other original parts.
So for example if the car has the 120,000 miles on it and the same engine and same transmission is in it that came from the factory and has the 120,000 miles on it with the same engine and transmission then that is what original miles refers too.
Basically the engine and transmission are still going strong and haven't needed a rebuild or replacement.
Now some front end parts, brakes etc could have been replaced during those times and during those miles.
But if the vehicle lists original miles on it then it means that the engine has not been rebuilt, the transmission has not been rebuilt or the and transmission has not been replaced.
Also when the vehicle says original miles it means that the means that the odometer has not been replaced, rolled back, or has not gone past the maximum limit.