"Offline" has two meanings here.         The first is that the archives are stored on your own media and not somewhere in the internet (like in a cloud or hosted on an internet server). So nobody should be able to remove the archives except you (but: see second meaning).          The second meaning is to store it on media that is not even connected to your computer (after storing). Like a DVD or an external drive, at best wrapped in tin foil against EMP. The external backup also saves your archives against deletion by a computer virus or any kind of remote control software. The most important archives should be stored on at least two different storage media as duplicates.