Example
pYou are the head of a programming team and you want one of your programmers to write a method for part of a project.
HERE ARE
THE REQUIREMENTS
FOR A METHOD THAT I
WANT YOU TO
WRITE.
I DON'T CARE
HOW THE
METHOD WORKS,
AS LONG AS THESE
REQUIREMENTS
ARE MET.
As an example, suppose that you are the head of a programming team.  Your team is writing a large piece of software, perhaps with millions of lines of code.  Certainly nobody can keep all those lines of code in their head at once (not even me!).  So, the large problem is broken into smaller problems.  Those smaller problems might be broken into still smaller problems, and so on, until you reach manageable problems.
Each of the manageable problems can be solved by a method - but you won't be writing all these methods.  The methods are written by members of your team.
As each team member is given a method to write, you will specify the requirements of the method by indicating what the method must accomplish.  But most of the details about how a method works will be left up to the individual programmers.