What exactly is Godwin’s theory? It seems related to discussions and mentioning of hitler/nazis, but I can’t seem to get it from the context it’s used in.
Eventually a poster in a thread will mention Hitler or the Nazis. At that point the thread is over and the poster has lost.
Just like Aro said. 
And if Aro’s link doesn’t answer your question fully this guy goes into an extruciatingly detailed description.
… Leading to this link where Godwin himself speaks … interesting stuff!
Julie