VCO (Voltage Controlled Oscillator)
In easy words
Oscillators are circuits that output a steady tone. If the frequency of this tone can be controlled by an external control voltage it can be called a voltage controlled oscillator. The waveform this oscillator outputs makes a difference in timbre and overtones.
Many eurorack oscillators output multiple different waveforms at once. The Input which controls the frequency is sometimes called 1V/Oct. This describes the sensitivity of the input: a change of 1 Volt in the input changes the output frequency for one Octave.