Here is the non-technical explanation.
The VCA is the actual unit that does the work when it comes to controlling the amounts of anything else. Like a volume control on your phone or car's stereo, it controls the amount of whatever it is you're putting through it.
But where a VCA differs from a simple volume knob is that it has an input for automation. Instead of turning the knob by hand, you can use a CV voltage to do it. VCAs can control the amount of audio. They can control the amount of other CV signals being routed through them. They are the basic building blocks of automation aka MODULATION.
There are some fine points regarding different types of VCAs and what they can do. VCAs can be set-up in many different ways... so you might find some that respond in linear or exponential fashion. You will find some with a manual knob. You'll find others that have no manual controls and are just three little jacks. Sometimes a set of VCAs will be built to do duty as a mixer as well.