The Voltage Controlled Amplifier (or VCA) is the module responsible for "making the notes" in an analog synthesizer.  Remember that VCOs do not turn-on or turn-off the waveform outputs.  It is the VCA that performs the "now you hear it, now you don't" function.  In the most basic implementation, it is an amplifier whose gain is adjusted by a control voltage.

A VCA has at a minimum, a signal input, a control input, and a signal output.  There is usually a control panel adjustment called "Gain".

The signal input assumes that it will get "audio" in the range of +/-5V (10 volt span) but most VCAs can also process signals down to DC.  The control input is always DC-coupled and is usually optimized for 0 to +5V control range although most will tolerate +/-5V control voltages.  If an envelope generator is connected to the control input, and a VCO waveform output is connected to the signal input, one gets a envelope modulated output.

For example suppose the VCO output is connected to the signal input of the VCA.  And assume that the envelope generator is connected to the VCA control input -and- the envelope generator output looks like this:

You will get an amplitude modulated output that looks like this:

As you remember from reading about envelope generators, the timing of the four elements of the envelope generator is highly adjustable.  By adjusting envelope timing, one can get the sound dynamics of a gunshot, drum hit, piano, reversed drum hit, bell, organ, or just about any other dynamic you might imagine (any many that you have not imagined).

The panel "Gain" control is used to modify the control input range, such that more or less control input is required to effect gain changes.  That helps to create "explosive" sounds in one extreme, or let some of the signal input "leak through" constantly for a special effect.