A message allows whatever is actually inside the box to communicate to the 'outside world'. Communication to an object (or between objects) is done via messages. Messages define the interface to the object and everything an object can do is represented by its message.interface  

For example, the message interface to print is a graphic (Graphical User Interface), representing a printer. Click on this and you can print. Why would you want to see the code that allows this? (Actually, you might if you wanted to change it...but more of that later) So, unless you want to change how a program works, you shouldn't have to know anything about what is in the box (object) just in order to use it. 

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