The remastering process is audio magic. Some remasters are considered to be amazing, and some are considered to be garbage. Remastering is a term used for when an original recording master, is digitized and the audio is cleaned up. This can remove noise from the background, it can help level the audio out some tracks, it can bring up or down certain vocals or instruments, etc. The process of simply digitizing the original analog audio, then releasing everything again is simple enough to call it remastered. But, typically more is done to try to improve upon the original release.
Keep in mind that many older releases were recorded onto tape. Analog. So, they are degrading. At some point, many of these have been put into the digital domain, so they no longer degrade, but they may still have some issues with noise and levels that can be corrected.
The quality of a remaster is entirely subjective. Kind of like those who listen to vinyl because they like the way it sounds... whatever that means. Some people will love a remaster, some will not. There is no 'right' answer on this. It's kind of like a Director's Cut of a movie. Maybe it is better than the theatrical release, maybe it is worse. But, that's as much opinion as it is fact.