There's no easy way to explain how multipass works. Basically it's just a name for any technique that requires the same mesh/scene to rendered more than once in order to get the desired visual result. One multipass technique could be blending.

For example, if you want to render a terrain with a number of blended textures, but your target graphics card doesn't support this many textures/stages, you can still achieve the result by partially rendering the terrain with a subset of the textures and blending the results of the passes.

As far as I've seen so far with my limited experience, multipass rendering mostly comes into play when your target hardware doesn't support rendering the visual effect you want in one pass (please correct me if I'm wrong). As such, there's little we can tell you about multipass rendering in general. If you're thinking about a specific application (effect/technique) you wish to use multipass rendering for, please post it, since that should make for a more interesing discussion.

Hope this somewhat answers your question.