Webmasters can also edit content through a graphical Control Panel that gives you complete control over your site.
This sentence refers to the Administrator Backend of your Joomla installation usually found at:
* yoursite.com/administrator
The graphical interface refers to the fact that you have things to click on and there is a nice layout...automated features so to speak. This, in computer talk is referred to as a "GUI" or Graphical User Interface. This is in contrast to having to sort through pages of code and using only a text editor to change them.
The edit button mentioned is something that you will see near the articles title if you are logged in to your sites front end and if you have proper permissions to edit articles.
Joomla Access Level descriptions
You can also assign a new menu item of a "Submit Article" layout...be sure to set this to "Registered" users and not public. This gives you the ability to submit new articles from the front end.
Some of the sample content in the Demo's is Joomla's sample content since it's all working together as one big system.