What does it mean when a song is remastered??

There's roughly 3 stages of music production. Tracking - Mixing - Mastering Tracking captures the original performances on multiple tracks. in the 50's, most records went straight to one or two tracks - mono or stereo. mix was basically done at the end of recording. in the 60's, they quickly jumped to 4, 8, 16, and 24 track tape machines. Then two 24 track machines working in tandem for 48 tracks. Audio Consoles with lots of channels followed. The combination of lots of tracks and lots of channels opened up new possibilities in mixing. The song could be further shaped after recording with EQ, Compression, and time based effects like reverb and delay. This process continues to this day, but generally with digital recordings, where a computer takes the place of a tape machine. After the song is mixed, it goes to MASTERING. The goal of mastering is to establish a unified loudness from the various songs, competitive loudness and overall tone, and to make small adjustments so the mix translates well to the final format, be it vinyl, CD, streaming, etc. Mastering engineers only have the final 2-track stereo mix. they cannot adjust the individual tracks, only the song as a whole. Its just more compression and EQ, but taken to a microscopic level. It also helps to have a different set of ears on the project. some songs might sound noticeably different after mastering, some might sound exactly the same. a REMASTER is when they take the original mix and re-do the mastering process, making it "more suitable" to the current listening environment. sometimes they address specific limitations of the time. most of the time they just make it louder and more compressed. in the 70's music was listened to on big speakers, even in the home. thats rare these days. songs are mastered for airpods on the train or at the gym, or car speakers, etc. those are noisy environments. loud and compressed is helpful in those situations. also, with people listening to playlists and not full albums, theres an incentive to make your song louder than others, because louder scientifically sounds better. hence the loudness wars that started in the 90's. play a record from the 70's or 80's that HASN'T been remastered, and chances are you will be reaching for the volume knob and cranking it way up. the 'downside' of making it louder is that theres an audio ceiling. The closer to that ceiling you live, the less room for dynamics. when everything is loud, nothing is loud. it can lose impact and be fatiguing.