A De-esser is mainly used on vocals , It reduces sibilant frequencies via volume, sibilants such as S's & T's are most common.
The De-Esser works like a compressor or a dynamic EQ, only it zones in on the selected frequencies, usually the frequencies are between 4-10khz but some plugins may give you more area to work in as shown below in the UAD version its 2-16khz.
With De-Essers usually being used on vocals there is nothing stopping you from using them on other instruments such as Hi Hats, It can be used on the mastering chain to control unwanted high frequencies.
Frequency - Allows you to select the frequency that you wish to use for the sibilants, between 2khz - 16khz on the above version.
Threshold - Activate's the De-Esser above the threshold set value, the UAD version above has a set ratio of 7:1 and a range between -40dB to 0dB.
Solo - Allows you to solo what is being affected by the de-esser, can also be used to zoom in on the frequencies whilst turning the frequency knob from 2khz to 16 kHz.
Speed - This is a set attack / release time, a bit like the compressor/ limiter. The version above is
Fast: Attack = 0.5ms, Release = 30ms
Slow: Attack = 2.0ms, Release = 120ms