Ever photocopy a photocopy? And then have you photocopied that photocopy again? Notice how the quality goes down each time? The same is true with any kind of recording.
The “master” is the original studio recording clips for a song. Every time they’re read from, that read will never be as high quality as the actual recording. That master copy is very rarely read from, since you always have the risk of damaging it each time you access it. Instead, there will be a version of the song created from it, and then that copy will be used to make more copies for eventually creating CDs/records/etc.
When a song is remastered, they go back to those original recording clips. This can either be just re-copying them with higher quality on that initial read, or they could be piecing the song together again, possibly from other takes of certain sections, or with slightly different timings on the transitions.
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