"Mastering" in its original sense is producing a master stamp for pressing vinyl records, the term has extended to recording raw tracks, tuning, mixing and compiling them into finalized tracks to be put on a master disc or tape ready for replication.

Remastering means the raw tracks are mastered again using newest technologies, like hi-res encoding or surround sound/spatial audio .etc.

This is a great comment by another Reddit user:

Remastering explained

It is a true art form and people like Kevin Gray and Bernie Grundman are living legends that need to ensure this craft is shared.

Studio recordings are usually multi-tracks that are combined into a single stereo mix. The producers and audio technicians did their best to create a master that would give the best result, which prior to about 1980 was vinyl records. Remastering means you go back to the original recordings and create a new master that is created to sound great on modern media and equipment. They might also clean up noise, and make other corrections like dynamic range and equalization.

Although frequently a remix is sold as a remaster, if the intention of the remix is to replicate the original work.

Mastering can involve making a new transfer from original tapes (in hopes of getting a better starting point than the last transfer), but mainly it is sonic enhancements: noise reduction, EQ, and dynamic compression/expansion. Judicious use of these tools (not just turning all knobs to "11") can result in a more polished, modern sound that makes each song sound as good as it can (in the engineer's opinion); and it can smooth out inconsistencies from song to song (so everything on an album sounds like it belongs together). *Re-*mastering is just when someone does all this work again, often just for the sake of being able to market the same music again as "improved".

The ELI5 version is more like:

You know how your audio player has an "equalizer" or "tone controls" where you can change the volume of, say, just the bass and treble pitches in the music? When you mess with those settings, you are, in effect, "remastering" the audio as it plays. Well, someone did that (and a few other things) on every piece of recorded music you've ever heard, after it was created, to make sure it sounded really "good", in their opinion. This was "mastering" the audio. At some point, when the record company wanted to sell more copies, they got someone else to do that work again, and they marketed the result as "remastered", implying it sounds even better now. Whether it sounds different, and whether it is actually better, is often a matter of taste.

Source: I have done mixing and remastering work professionally.

This can often be a problem as newer remixes often reduce the dynamic range in older pieces

But what form did the original recordings take?

tape, if we're talking vinyl era

I assume those kinds of tapes have a lossless quality. Compared to those crappy cassettes I had in the 90s?

Better than crappy 80s cassettes yeah but not lossless. One of my favourite albums is Fresh by Sly and the Family Stone and that famously sounds a bit weird because apparently perfectionist Sly overdubbed so obsessively he wore the tape thin

Please read this entire message

Your submission has been removed for the following reason(s):

• ELI5 requires that you search the ELI5 subreddit for your topic before posting. Users will often either find a thread that meets their needs or find that their question might qualify for an exception to rule 7. Please see this wiki entry for more details (Rule 7).

If you would like this removal reviewed, please read the detailed rules first. If you believe this submission was removed erroneously, please use this form and we will review your submission.

When they’re brand new and in good shape, they’re not lossless, but they are about as good as you can get in the analog world. The problem is that the tapes are a physical media and will deteriorate over time. Or, even worse, they might get destroyed either by accident (like in a fire) or on purpose (like when they destroyed episodes from “Dr Who” and “the Tonight Show”).

In the 1960s, Ampex developed 16 and 24 track 2" wide tape recorders that revolutionized the recording industry. They go back to those recordings to do the remaster.