A coinage dating from Usenet days, Poe's Law states that without an explicit indication—such as say a winking smiley—that a parody of a given crackpottery is, in fact, a parody—that it is impossible to tell it from the real thing. This is a noted and general problem in fora discussing pseudosciences and religion.
In colloquial usage, a 'Poe' may also refer to both (i) such a deliberate parody, and, logically (due to the ramifications of a law) (ii) any material having the quality that it is difficult to determine if it is or is not such a parody.
See also RationalWiki's explanation.