Picture a paperboy in the Great Depression wearing a sign that engulfs his body, blaring out the headline.
Picture Bruce Willis in Die Hard with a Vengeance, wearing a sign that says (essentially), "I hate blacks," in Harlem.
Picture anyone wearing a sign. A pair of large panels with bold writing, and a pair of straps for over one's shoulders. You become a walking billboard. Your freedom of speech is amplified. Your message is, at once, more and less credible.
That's a sandwich board. Before you dismiss anyone wearing one as crazy, ponder their message, and take a moment to appreciate the First Amendment. You don't see many sandwich boards with deviant thoughts in Belarus or China.