Before the advent of MIDI, there was only one realistic method for interfacing all of your pieces of equipment with each other. Control voltage, more commonly known as CV and more accurately known as CV/Gate, is a system that uses very simple analogue electrical signals to trigger equipment.
The CV/Gate system has two parts: the control voltage itself and a separate gate (aka trigger) signal. Both trigger a note, but the CV element can be thought of as the part that determines the pitch, while the gate determines the note on/off timing. CV control needs two separate signals, often transmitted over two cables for flexibility.
