I'm going to put this in simple terms, not because of you, but because I'm not an engineer, either. Generally speaking, an amplifier needs a stronger signal than that which is put out by a passive bass. Therefore, what a preamp does is take the signal from your bass, and make it strong enough to go into your amplifier section. That's the only necessary part of a preamp.
However, most preamps have various tone controls such as bass, treble, and middle adjustments. This is called equalization, or EQ. That allows you to affect the tone of your bass, and make it sound the way you want. Some preamps also allow adjustment of the strength of your input going into your preamp, which allows you to adjust for passive or active basses, or allows you to overdrive the preamp some to get a more aggressive tone. This is also called gain on some preamps.
A preamp connects to several things. The wall, for power. Your bass, to get the input signal. It also connects to the amplifier, to send that signal on its way to eventually get to the speakers.
So, in summary, the role of most preamps is to boost your signal, and to modify it with EQ. You should also note that many things out there commonly referred to as "amplifiers" or "heads" often have both the preamp as well as the amplifier section all in one unit, so that's all you need to plug into and that's all that gets plugged into your speaker. In fact, I would guess that many more heads are sold than separates to your common bassplayer. If the preamp, amp, and speakers are all three in one unit, that's called a "combo."
