The preamp takes a signal from a source, such as a CD player, processes that signal, and sends it on to the amplifier. One example of signal processing is splitting it appropriately among two or more speakers, for stereo or surround sound.
In a basic home audio system, the preamp is often in the same box as the tuner and amplifier, the entirety of which is called a receiver. Higher-end preamps are often combined with a tuner, but left separate from the amp. The less stuff you have in one box, the less likely you are going to have components interfering with each other.