This is what you see on cars with incredibily dirty rear windows. What happens is that some kid will use his finger (and, believe me, it's almost inevitably a boy) and write out "Wash Me!" or maybe "Please Wash Me!" on the back. It's kind of funny unless it happens to you.
The really clever kids write "Wash Me!" backwards ("!eM hsaW") so that it's legible through the rearview mirror.