Content is the actual articles and some of the navigation pages of Wikipedia, as opposed to administration pages such as talk, user and maintenance pages. Content is contained within the Main namespace.
Writing content is the aim of the process of building Wikipedia, although in order to do this, pages in other namespaces are needed.
The automatic definition for an article used by the software at Special:Statistics is: "any page that is in the article namespace, is not a redirect page and contains at least one wiki link." Content then is articles, redirects and disambiguation pages, all of which are for the reader rather than the editor.