In explanation of tag W3SCHOOL :
The tag defines external content.
What's the meaning of external content exactly ? It's mean quoted (from somewhere) text ?
From the HTML 5 specs:
The article element represents a component of a page that consists of a self-contained composition in a document, page, application, or site and that is intended to be independently distributable or reusable, e.g. in syndication. This could be a forum post, a magazine or newspaper article, a blog entry, a user-submitted comment, an interactive widget or gadget, or any other independent item of content.