Most filters can emphasize the frequencies just before those that are reduced in volume. Resonance is the degree of emphasis that occurs. It makes the filter much more audible and gives it character - it would otherwise sound like just altering the equalisation - and is the foundation of the acid sound....
VCS3
Very early VCS3 (pre-Putney), note Oscillator 1
The original portable synthesizer introduced in 1969 had a solid Aformosia cabinet
which housed the following modules interconnected by means of a matrix patchboard.
3 x Voltage Controlled Oscillators.
Noise Generator.
2 x Input Amplifiers.
Ring Modulator.
Voltage Controlled Low Pass Filter.
Trapezoid Envelope Generator.
Voltage Controlled Reverberation.
Meter.
2 x Stereo Output Amplifiers.
Joystick.
The VCS3 had:3 Voltage Controlled Oscillators, a Noise Generator, 2 Input Amplifiers, 1
Ring Modulator,1 Voltage Controlled Low Pass Filter,1 Trapezoid Envelope Generator,
a Voltage Controlled Reverberation unit, 2 Stereo Output Amplifiers and a Joystick.
Routing the various functions of the instrument was done via a small pin patchboard.