Top-down design is an approach to problem solving that takes the goal and breaks it down into a series of subgoals that lead to the solution.
A hierarchy chart can be used to describe how this is done in a visual form.
Finding The Hypotenuse
The following diagram shows the steps involved in working out the length of the hypotenuse of a right-angled triangle given the lengths of the other two sides. The top box shows the problem that we are solving. From left to right, the steps that lead to the solution can be read off in order on the bottom row of the diagram.
With a more complex problem, you might break down the subgoals into subsubgoals adding rows beneath the current layer of subgoals.