Sibilance refers to the harsh, hissy sounds that occur when a vocalist pronounces consonants like "s", "z", "sh", "ch", and "t". These sounds are characterized by high frequencies and can become glaringly loud and distracting in a recording, especially if the vocalist is naturally prone to sibilance or if the microphone and preamp used are particularly sensitive to those frequencies. Excessive sibilance can make a vocal track sound harsh, fatiguing, and amateur, and it can also clash with other mix elements making it difficult to achieve a balanced, polished sound. De-essing is the process of taming these harsh frequencies to make the vocal sit better in the mix without sounding dull or muffled.