'Protected' pages can only be edited by the chief editor.
Article pages are protected, by the chief editor, once they've been judged, by the chief editor, to have reached a certain minimum level of quality. The content of such pages is still open for discussion, but the chief editor has the final say about any changes. This is to preserve the quality of such pages.
However, while the site's content is at an early stage of development, and there are a relatively small number of articles, even some early-stage articles are protected, in order to help preserve at least the current level of quality on the site.
Non-article pages, such as help pages and contents pages, are always protected, given their critical function, although their content is similarly open for discussion.