Superimposition is a simple rule based on the sequence of events in the build-up of deposits on a site. As new layers accumulate, the older deposits become progressively more deeply buried. This allows us to excavate from the top down, going backwards in time through the archaeological deposits.
There are many ways that superimposition can occur, and these can be confusing. Factors such as removal of material by natural or man-made erosion, the digging of pits, ditches and post-holes (and in towns - cellars), and the re-deposition of soil originally from lower down, at a point higher up in the strata, can mean that the situation is a lot more complex than a simple top-down sequence. Sometimes we find ourselves digging something which actually belongs to an earlier period than the layer below it. An example of this is where a wall has collapsed many centuries ago, over a deposit (such as a household midden) which accumulated against it when the wall was still standing. The wall is actually earlier in date, but due to the collapse and spread of its building materials, it is now physically on top of the later layer. Disentangling these relationships is part of the challenge and fun of archaeology.