Ok, to be brief, a pre-amp is exactly what its name suggests. Its a small amplifier for the imcomming signals. It increases a signals strenght so that the amplifier stage has a nice strong healthy signal. I'm sure other people will be a bit more technical with it, but it's still the same idea. You would need one when the incomming signal is weak, like a mic or phono. Most receivers have these built in for the inputs labeled mic and phono.