Zuordnung
Mapping here means a rule that says which number (its usually about numbers) from one set belongs to which number from another set. Here is an example.
You have a set of cuboids of different sizes. Each cube has a weight. Each size of a cube corresponds to a certain weight. A possible rule could be:
Take a cuboid. Measure its lenght, width and height. Multiply the three numbers. Take the result and multiply it by 1.5. The result is the weight oft the cube in grammes.
This rule is called a mapping (or Zuordnung) in German. A mapping always tells you how to find a corresponding number to any other given number.