The best thing about some of the introductory concepts in geology is how practical and sensible they are. Superposition is no different. Superposition states that in an undisturbed section, younger material rests atop older material. How awesomely simple is that? Very.
To put it another way, in a section that has not been flipped upside-down (via tectonics), the oldest material will be at the bottom and the youngest material will be at the top. Or "stuff won't float in mid-air waiting for material to be deposited under it", which is how my TA phrased it.
The implications for this principle is that it is a tool that allows us to make relative statements about a sequence of events. If we have a mudstone at the base of an undisturbed section, and a sandstone at the top, we can make the statement that the mudstone was laid down before the sandstone. More importantly, we can talk about the depositional environment changing between two time periods, from one that favored deposition of mud, to one that deposited sand. We can't say anything specific about the timing of the events yet (except to say the mudstone is before the sandstone), but this relative sequence of events is a start to understanding a locations geologic story.
Here is a basic figure of the concept (click to animate): The first, and oldest layer, is the gray layer. This is overlain by the blue layer (the second oldest). And the overlying purple layer is the youngest. See, geologic concepts are, generally, very practical.
Edit: On a similar note About Geology has many excellent posts along these lines. Most recently, one on How to Look at a Rock.