Flow diagram of a feedforward compressor. It's also possible to do the level detection after the gain is applied (a feedback compressor, rather than feedforward).

The compressor is an example of a VGA - Variable Gain Amplifier - similar to the VCA - Voltage Controlled Amplifier of the world of synthesizers, but with one important difference. A VGA "listens" to the input amplitude of a signal, often according to the Root Mean Square (dB RMS) of the amplitude - and as a result "averages" the amplitude in much the way the ear does. Therefore, the dynamics processor can be seen as a tool that extends human hearing in an appropriately musical manner and that provides a means of making instantaneous changes in gain automatically.

Root Mean Square comes from mathematics. It is the square root of the average of the squares of a group of numbers. A more useful and meaningful way of averaging a group of numbers. It is a statistical measurement of amplitude in this case.