To what extent may someone use the content of a published paper in an expository piece on that paper?
From an ethical point of view, you must cite your original source appropriately. Clearly indicate 


when you are using ideas, words, or figures from another work by citing the source 
when you use something verbatim (typically by putting it in quotation marks or blockquotes)


As long as you attribute your sources properly, there isn't really an ethical problem with excessive use of a source.

From a legal perspective, it can be trickier. In summary,


Copying large amounts of text verbatim: may violate copyright
Copying small amounts of text verbatim, for academic purposes: typically allowed under fair use, though like everything legal, this varies by jurisdiction
Paraphrasing text: does not violate copyright. The intellectual content of a paper is not subject to copyright (as opposed to the words used to present it, which are). 
Reusing figures: may violate copyright. Check the policy of the source's publisher.


From a "best practices" standpoint, work that involves an excessive amount of paraphrasing is often lacking original intellectual content. Even if your work is expository, you're supposed to be adding some value for the reader, not just stating the content of the original source in other words. So be careful of this, too :)