OK, here goes. Sorry if any of it is patronising.
Whether a stereo or a surround sound design, an amplifier has two roles to perform: one, to interpret the audio signals being fed to it (either music, or movies, or both), and two, to make said signals louder. An integrated (one-box) amplifier performs both functions inside one chassis. A preamplifier and power amplifier combination separates the two roles out into two boxes: the first, the preamplifier, is the 'brain', the second, the power amplifier, is the brawn.
To connect a preamplifier to a power amplifier, you typically use phono interconnects, as you would use to link a CD player or tuner to a standard hi-fi amp. How many you'll need corresponds to how many channels of power amplification you want to use, which in turn tends to reflect how many speakers you have in your system. So, for a five speaker home cinema system, you'll need five phono interconnects to feed five power amp channels, one for each speaker.
Preout sockets are simply the connection sockets for these phono interconnects. A preamplifier will always have them -ÿthe total number it offers will reflect whether it's a stereo design (two sockets), a 5.1 design (six sockets, one for a bass-generating subwoofer) or a 7.1 design (eight sockets, one for a sub). You'll find that most surround sound amplifiers offer a similar socket arrangement - although has Andrew has pointed out, that's not always the case.
In the latter case, the feature is designed to allow an upgrade path for owners of integrated home cinema amps. You could buy your modestly powered receiver now, then add a bigger, more powerful amplifier later to increase its output, or conceivably to investigate biamplifying (ie using two power amp channels per speaker) your system. It's also possible to connect an active (self-powered) speaker directly a preout socket, should you want to.