In MediaWiki parlance, a page is said to be protected if an administrator has specifically marked it as being unavailable for editing by anyone without the privilege to protect it. The edit tab for protected pages is normally replaced by a view source tab.
For a list of protected pages in this sense, see Special:Protectedpages.
For policies and guidelines regarding page protection, see Protected page guidelines.
On QED there are various ways to restrict read and/or edit access. In particular, see: