The story is that Prince John (remember him? long time ago - King Richard, Robin Hood and all that) wanted to visit - or at least pass through - Gotham, which is a village in Nottinghamshire. The good folk of Gotham thought, "Ooh, don't fancy that. All that mess and kerfuffle to greet a king." Or words to that effect. Anyhoo, they decided to pretend they were all idiots - the picture shows them rowing a boat in the sand - and, sure enough, King John said "Coo, they're all nuts, I'm not going there, let's build a bypass".
So the moral is that sometimes you have to be a fool to be wise. Actually, that probably isn't the moral, but it rounds off the story.