It's the input signal voltage to the amplifier. A very cheap amplifier might be only 1V or 2V but the vast majority of amps are 4V or above. Ideally you want a variable input voltage, so you can calibrate the amp to your head unit (so 0dB attenuation from the empeg matches the max gain of your system).

My experience is that even cheap amps have variable input up to 4V.

Rob