LUFS (Loudness Units relative to Full Scale) is the standard measurement for perceived loudness. -14 LUFS matters because it is Spotify's default playback level: in Normal mode, Spotify turns louder songs down to about -14 LUFS. YouTube uses a similar reference, and Apple Music's Sound Check plays music at about -16 LUFS.
-14 LUFS is a playback level, not a mastering target. Turning a loud master down is transparent, so it loses nothing. A quiet master is turned up only as far as its peaks allow, or not at all, so it can sound weak next to other songs. Most pop, electro, and metal releases are mastered around -8 LUFS, and rap, RnB, and rock around -9.5 LUFS, with true peaks at or below -1 dBTP. Jazz, classical, and other dynamic music can stay closer to -14 LUFS. See the best LUFS for AI music by genre.
Use the Loudness Penalty Analyzer or LUFS analyzer to see how much each platform will turn your file down, then use Automatic Mastering to reach your target safely.