The mapping principle states that if your user interface relates to a spatial arrangement of objects in the real world, the user interface elements should be arranged similarly to the real world objects.  That way, the user can easily figure out which element corresponds to which object.  For example, the explanatory labels for the two art pieces below are arranged the same way as the pieces themselves making it clear which label goes with which piece:

The principle can be easily extended to other physical properties, such as color, size, direction, etc.  Elevator call buttons, for example, are typically arranged vertically making it clear which button should be pressed depending on what direction the person wants to go. And in the European toilet design below, the buttons for triggering large/small flush are correspondingly large/small.

Examples related to this concept:

Further reading: