preamps are the RCA plugs on the back of your head unit that you plug amps into. preamp means before amplification (i.e., your built in head unit amp wont touch that signal, because a double amplified signal could be bad news).
most of them are between 2 and 3 volts. i really dont know what advantage a higher voltage is. you have to match your amps gain to that voltage though, to avoid clipping the signal and putting your amp and speakers at risk.
that was my layman's explanation ;D