Loudness Units Full Scale

A peak meter tells you how close a signal comes to clipping. LUFS tells you how loud it sounds to a person, measured over time and weighted for the way hearing responds to different frequencies. Integrated LUFS is the single figure covering a whole video.

Platforms measure what you upload and turn down anything above their target, so mastering louder than everybody else no longer buys you volume. It gets attenuated, with the dynamics you crushed to get there still gone. Consistency across your own catalogue matters far more than hitting an exact figure.