The HPF is a high-pass filter; it allows for cutting the low frequencies. By adjusting the CUTOFF slider, frequencies below the cutoff frequency will be cut.
The VCF (Voltage controlled filter) is the low pass filter section.
The CUTOFF slider set the cutoff frequency point. Anything above the cutoff frequency will be cut.
The RES slider controls the filter resonance, which amplifies frequencies around the cutoff frequency.
The SLOPE switch allows for selection of the slope for the low pass filter.
The LFO MOD slider allows the use of the LFO to variate the cutoff frequency.
The ENV MOD slider adjusts the depth by which the envelope controls the cutoff frequency.
The ENV-1/ENV-2 swicth selects the envelope which is used for control.
The KEY FOLLOW slider permits the cutoff frequency to be controlled by the keyboard.