The audio spectrum represents all of the frequencies a typical human can hear. On average, we can hear between 20Hz to 20,000Hz; Hz is short for Hertz which means vibrations per second.
Any frequency below 20Hz we tend to hear as a rhythmic click of some sort or as a vibrating sensation depending on the sound source. Frequencies above 20kHz we feel as pressure in our heads, but often can’t exactly hear the note in a traditional sense. Every human has a different hearing range that will change throughout their lifetime due to hearing loss.
If you want to pursue music professionally it is of the upmost importance to protect your hearing and get regular hearing checkups!
Saturation happens when a vast majority of the spectrum is filled with noise of some kind. In other words, many frequencies, from all over the spectrum, are being used at the same time. This is common in many popular styles of music including: rock, metal, pop, techno, dubstep, hip hop, etc. Some common terms for moments of saturation in popular music include “bass drop” and “breakdown.”
In some ways we can look at how the shape of the audio spectrum changes throughout a song as a way to map the form of a song beyond “song form” terminology. With this mindset, moments of spectrum saturation are critical, climatic moments in a song.