Loudness Normalisation

LUFS is the unit here, measuring perceived volume over time instead of peak level. Playback systems compare a video against their target and apply a gain adjustment, which for most modern masters means quieter than you exported. Right-click the YouTube player, open stats for nerds, and you can read the content loudness figure for any video.

What this changes for a creator is that squashing a mix with heavy compression to sound louder gains nothing once playback pulls it back down. You have spent your dynamic range for a result nobody hears. Mix for clarity instead.