What is "de-essing"

De-essing is any technique intended to reduce or eliminate the excessive prominence of sibilant consonants, such as "s", "z" and "sh", in recordings of the human voice. Sibilance lies in frequencies anywhere between 2–10 kHz, depending on the individual voice.

Excess sibilance can be caused by compression, microphone choice and technique, and even simply the way a person's mouth anatomy is shaped.