Poe's law states: “Without a clear indication of the author's intent, it is difficult or impossible to tell the difference between an expression of sincere extremism and a parody of extremism.”
It is an observation that it's difficult, if not impossible, to distinguish between parodies of fundamentalism and other absurd beliefs, as well as their genuine proponents, since they both seem equally insane.
Poe's law applies not only to the absurdity of beliefs, but also to the absurdity of the arguments that are used on behalf of those beliefs. Arguments on behalf of young-Earth creationism and theodicy are especially known for their absurdity.
It is important to note that: linking a claim to Poe's Law is not the same as suggesting that said claim is in fact any type of parody at all. On the contrary, linking to Poe's Law just means that you could not tell if said claim was parody, or indeed sincerely held crankiness — assuming the original claimant didn't decide to also supply a clear indication of intent. When one is presented with a claim that is so over-the-top as to either be a brilliant parody or reflect a genuinely outrageous extremist belief, Poe's Law has been invoked.