A VCA or "voltage controlled Amplifier" is a module which does exactly what it name says: you can amplify signals via voltage control. You can use a VCA to control the Level of every signal in your Modular system via another signal. In radio electronics this is also known als "Amplitude Modulation" as it modulates the amplitude of the signal: AM.
Here is a typical (Quad-)VCA:
It has four inputs and four outputs (below) and a CV-Input for each of the four VCA circuits. With the level knob the base level of each VCA can be set (this way it acts very much like the typical mixer). The CV has a little knob to attenuate the incoming envelope and a LED to display it. Additionally the response of the CV input can be changed anywhere inbetween linear and exponential with an additiona knob. Usually linear response is more comfortable when editing CV signals, where exponential response is more useful when editing audio signals.
There are different ways of implementing a VCA. Eliott Sound Products has a good guide on the various techniques to achieve a VCA called VCA Techniques Investigated.
A small overview over the pros and cons of the different technologies:
Speed (Attack/ Release)
THAT2181, SSM2164, ...
AD633, MC1594, AD834, ...
¹ usually adjustable using preset pots
² depends on speed and circuit topology
³ speed depends on implementation and topology