A master is the single original copy of a medium by which all other copies would be derived. Before the 1970s and the advent of digital audio, remastering, or creating a new master copy, meant the loss of valuable quality or "generational loss."
When a song or collection of songs is digitally remastered a conversion of the music takes places where the original analog version is transferred to digital format. Though the term is widely associated with the digital audio age, the process of remastering music has been in existence since the recording of music.
Remastering a movie involves the enhancement of both the quality of sound and picture. Often it means a transferring from one medium to another, but this is not always the case.