They indicate that a) there's a neighborhood watch and b) curiously pointed silhouettes in trench coats and fedoras are not welcome.
That's all well and good, but in the Twenty-First Century, who's really afraid of bumping into James Cagney in the middle of the night? That being said, some of you may think the sign, and perhaps the whole idea of a neighborhood watch, is a little out of date. Well, I beg to differ. Take a look at who's behind the mask.
Homestar?! Well it wouldn't have been my first guess, but he's got to be at least as bad for the youth as cigarettes and dice. I guess each generation has its own cross to bear.