Loudness normalization is the automatic gain adjustment that streaming services and broadcasters apply so that one track does not sound much louder than the next. Rather than looking at peak level, it relies on integrated loudness as defined by the ITU-R BS.1770 recommendation, expressed in LUFS (loudness units relative to full scale).
Targets differ by context. European broadcasters following EBU R 128, first published in 2010, aim for an average of −23 LUFS. Spotify adjusts tracks to about −14 LUFS by default, with Loud and Quiet settings of −11 and −19 LUFS for Premium users. Louder masters are simply turned down, while quieter ones are turned up only as far as their peak headroom allows, and when an album is played in full it is adjusted as a single unit so that quiet songs stay quieter.
The practical consequence is that pushing a master to be as loud as possible brings no advantage on these platforms and can cost dynamics. Jazz lo-fi, with its soft transients and relaxed mixes, usually benefits from a master that breathes.