The VCF section is classic 24 dB/oct lowpass "ladder" filter. If you're not familiar with how voltage-controlled filters operate, a lowpass filter (LPF) removes high frequencies as its cutoff frequency setting is decreased from max, resulting in a dulling of sound.
Frequency- Sets the point where high-frequency attenuation begins.
Resonance- Emphasizes sound energy at and around the current cutoff frequency by adding feedback from the filter's output back to its input. At lower settings, this can be used to create mild resonances such as those heard in acoustic instruments. When combined with moving or modulating the Frequency slider, higher Res settings will give the familiar analog synth "wah" sound that launched 1000 disco records (or synthesized mute trumpets).
Filter Invert- There are two audio paths into the VCA - one from the VCF and the other from the resonator banks. This creates a number of frequency selective phase reinforcement and cancellation effects. Engaging the phase button inverts the VCF and effectively doubles the number of available timbres. Relative gain, mentioned above, also affects this. If the VCF is the only source into the VCA, Filter Invert will have no effect on sound.
VCF Modulation Sliders
ADSR- Sets the amount of modulation of cutoff frequency via the ADSR envelope generator.
AR- Sets the amount of modulation of cutoff frequency via the AR envelope generator.
LFO- Sets the amount of modulation of cutoff frequency via the low-frequency oscillator.
Growl (Grwl)- Sets the amount of modulation of cutoff frequency via the specialized "growl" low-frequency oscillator. This LFO used a unique waveform running at a frequency of 32 Hz, and is normally used in conjunction with the Touch Sensor Growl mod routing.
Velocity (Vel)- Sets the amount of cutoff frequency modulation via keyboard velocity. Higher slider settings lower the initial cutoff frequency, and playing harder = higher cutoff frequency/brighter filter tone).
Keyboard (Kbd)- Commonly known as "keyboard tracking," this causes the cutoff frequency to increase as ascending notes are played on a keyboard. The idea behind this is, because actual note frequencies rise as higher pitches are played, the Kbd slider adds a CV from the keyboard to the cutoff frequency in order to maintain the brightness of notes as higher pitches are played.