Your web server will let any visitor read the contents of that folder. If you put your code in there, then other people could read your code. When you were using php, this was ok, because your web server knew that it should not let people read your php files, but instead run the file and serve the output of the file.
With python, it's not the same. Your Python application acts as a whole. People aren't calling specific files like http://www.ggg.com/manage.py
They request a page from your server. The server knows how to use a wsgi file. The wsgi file runs your code, and tells it what the user requested. Your code returns a response, which the web server sends back to the user.
This code can live in any folder on your server, but if you want to keep people from seeing your code and database credentials, then it should not be in /var/www, but somewhere else. The book suggests your home folder. If you don't know what that means, find a tutorial on basic command line or basic Linux.
Good luck.