Mastering takes a finished stereo mix, or a set of them, and prepares it for release. Traditionally it was the step that turned separate recordings into a coherent album: corrective EQ and dynamics processing give the tracks a consistent sound and help them play well on the widest possible range of systems. Format-specific work can include controlling transient peaks or keeping the bass in mono for vinyl cutting.
Streaming has changed one priority. Spotify, for example, adjusts tracks to −14 LUFS by default, so pushing a master ever louder no longer makes it louder for most listeners and can cost it dynamics. For jazz lo-fi, which depends on soft, breathing dynamics, a gentle master usually suits the music better than a heavily limited one.