Remastering is when a new master is made for an older release, giving it an overhaul in quality and preparing it for distribution on current platforms. Improvements should include reduced noise, repaired errors, and sharper resolution, on top of the usual improvements expected from mastering.
Ideally, if the original mixes are available, they are the best source to work from, allowing the most to be done. We can also transfer from analog sources. In some cases, working from the original masters is also possible. Still, it depends on how those masters were treated originally.
To some, remastering is not always seen in a positive light. Personally, as fans, we've been disappointed by some. Here at Resonance Sound, we don't approach remastering as louder and more hyped being better. We seek to enhance the material while being faithful to what made the original recordings great. Moderation is the key!