A preamp is a VERY small amplifier that usually has active tone controls (boosts and cuts frequencies). also if installed on a bass/guitar it eliminates excess capacitance that a long or cheap instrument cable will introduce into your signal chain. preamps can also be it pedal format and stand alone or rack mounted and be very simple or very complex. every amp except a power amp has a preamp already.
You can google Preamp and find the mistakes I made also
I also agree with the Guitar Center trip, but first you can search there web site for used gear and search your local store.
hope I helped