What

A modular synthesizer breaks out the functional blocks of a synthesizer in individual modules, which can be connected to each other with some standardized interface. This means they can be rearranged and connected in different ways. Modulation in modular synths refers to using a signal or value from one module to control a parameter of another module or function. These values are transmitted as voltages over patch cables, and are also known as Control Voltage (CV).

Eurorack is a mechanical and electrical standard for modular synths. In eurorack, all signals are in principle the same (with some caveats), meaning that audio signals can be used as control signals, and vice versa.

Audio signals are generally ±5V. CV can be basically anything from ±10V, but often 0-10V or ±5V. Signal inputs will have limited voltage- and frequency ranges that they respond to, but signals outside the range will usually just get "ignored" or rounded to the nearest valid value. There are generally protections in place to prevent any excessive voltage to cause damage, as long as it's within the ±12V range of the power standard. Accidentally connecting an output to an output will also generally not cause any damage, typically an output resistor of 1k is used. The pitch control standard is 1V/octave.