ReplayGain is a tag-based loudness-normalization scheme: analyse a track's loudness once at encode time, store the recommended gain adjustment (and a peak value) as metadata in the file, and let any compatible player apply it on playback so songs sit at a consistent level. It predates today's LUFS-based platform normalization but solves the same problem — the jarring jump in volume between tracks from different sources — without touching the audio itself, which keeps it reversible and lossless. It offers both track gain and album gain, so you can preserve a deliberately quiet interlude within an album. Conceptually it is the ancestor of the loudness normalization every streaming service now does server-side.
