Yoad Nevo opens his de-essing tutorial by laying out exactly what sibilance is and why it causes problems in a vocal mix. Sounds like s, z, sh, t, and d produce sharp bursts of noise that pull the listener's attention away from the emotion of a performance and toward the technical fact of a microphone recording.
The distinction between voiced and unvoiced sibilants matters for treatment. Unvoiced sounds like s and t are pure noise, while voiced counterparts like z and d engage the vocal cords alongside the noise burst. That overlap of high-frequency sibilance and upper vocal harmonics is what makes de-essing tricky in practice.
Yoad closes the segment with a vocal sample run through basic compression and EQ, making the sibilance audible and concrete before any plugin solution is introduced.