The Deesser effect reduces 'S' sounds in vocals in a very unobtrusive manner.
De-Es controls the amount of attenuation applied to 'S' sounds. Applying too much attenuation will result in unnatural sounding vocals.
Threshold controls the level above which the Deesser becomes active.
Range controls the dynamic range the Deesser operates on. The threshold can be made to float within a certain range, so low-level parts can also be effectively deessed.
Frequency controls the frequency above which 'S' sounds are detected. If this control is set too low the Deesser will be too sensitive (i.e., sounds that are not a 'S' will be attenuated). By engaging the Monitor Button you can hear the filtered signal the Deesser uses. You can uses this option to judge whether the Frequency knob is set up correctly (ideally you only hear 'S' sounds and nothing else).