Reduce excessive sibilants (/s/ /t/ /sh/ etc.) in vocals and speech

Thresh Set maximum level for high frequencies
Freq Lowest frequency affected (a HF shelf is used for stronger de-essing than designs that work on a frequency band)
HF Drive Boost high frequencies before de-essing - can be used to enhance breathy vocals

For stronger de-essing you may want to use two plug-ins in series, or apply processing twice.