It will be more complicated than that. You'll need to modify the database schema, the DAO for the Article object, and the Article object class itself, in order to provide accessor methods. The ArticleDAO.inc.php file contains the SQL for accessing the article table in the database, and for assembling the Article Object (see _*FromRow method in there). The Article.inc.php file contains the Article class definition. You'll probably want to add a getSomething() and setSomething($value) method there.
In the Handler that generates the page you want to see or set that value on, you'll probably want to find the Form class that it references and add the necessary code bits to the readInputData(), fetch() and execute() methods in order to set the field on the form and then retrieve it. Finally, you'll need to modify the appropriate Smarty template that the form uses in order to display your value and let an editor set it.
The database schema is defined in a set of XML files located in the dbscripts/ directory. It's important to include your changes here in case you ever decide to run the upgrade tool. If you don't, you'll lose your new column.
Hope that's useful.
Software Developer, Public Knowledge Project