BATMAN FACT: The Batman distress signal made its first appearance in Detective Comics #60 in February 1942.
From Batman Daily Strip October 28, 1943
So if you see this symbol in the sky, in the dark of night, then you know the Joker or some other dastardly villian is wreaking havoc on some poor souls.
However, we can rest easy for we know Batman will soon come to rescue and save Gotham City.