LUFS, true peak, and what loudnorm actually does
This tool runs FFmpeg's loudnorm filter, which measures your track's integrated loudness in LUFS, a unit built around how loud human hearing perceives a signal averaged over time, then applies gain so the result matches your chosen target. That target is separate from the true-peak ceiling, which caps the single highest point the waveform reaches, including inter-sample peaks a normal peak meter can miss but a real digital-to-analog converter can still clip on. A file can be correctly normalized in LUFS and still clip on some devices if the peak ceiling is set too high.
This is a single-pass loudnorm run, so the filter estimates the needed gain as it goes instead of scanning the whole file first for an exact correction. That keeps it fast enough to run entirely in a browser tab, and it lands close to the target on most tracks; very short clips or material with extreme swings between quiet and loud sections are most likely to end up slightly off the number, worth a quick listen to confirm.