Often old recordings lack definition and loudness, as the recording analogue devices and processors were not in tune with the characteristics required. Thanks to the new technologies, we can now achieve a greater spectrum of sound.
Remastering is the process of making a new master for an album, film or any other creation. It tends to refer to the process of porting a recording from an analogue medium to a digital one, but this is not always the case.
For example, a vinyl LP originally pressed from a worn-out pressing master many tape generations removed from the “original” master recording could be remastered and re-pressed from a better condition tape. All CDs created from analogue sources are technically digitally remastered.
The process of creating a digital transfer of an analogue tape remasters the material in the digital domain, even if no equalisation, compression or other processing is done to the content. Ideally, because of the medium higher resolution, a CD or DVD (or even higher quality like high-resolution audio or hi-def video) release should come from the best source possible, with the most care taken during its transfer.
Additionally, the earliest days of the CD era found digital technology in its infancy, which sometimes resulted in poor sounding digital transfers. The early DVD era was not much different, with copies of films frequently being produced from worn prints, with low bitrates and muffled audio.
From any medium you have( old acetates, vinyl, tapes, etc..)
To any new medium such as Vinyl, lossless, Wav, Mp3 or recording.