There's none. Sources say that "Hitler Melon" is a computer animation mixed with real footage, showing Adolf Hitler eating a watermelon.
It was a made up video showing Hitler eating on a slice of watermelon during a speech. And some people find this funny.
"Hitler Melon" was also heard in an episode of American Dad called Camp Refoogee.
Check out 10 unknown facts about Adolf Hitler from The Crazy Facts.