Peak normalization is an automated process that changes the level of each sample in a digital audio signal by the same amount, such that the loudest sample reaches a specified level. 

Normalizing is indistinguishable from moving a volume knob or fader. The entire signal changes by the same fixed amount, up or down, as required. But the process is automated: The digital audio system scans the entire signal to find the loudest peak, then adjusts each sample accordingly.