What is a Bat-Signal? 

The Batsignal (or Bat-Signal) is a distress signal device appearing in the various interpretations of the Batman mythos. It is a specially modified Klieg searchlight with a stylized symbol of a bat attached to the light so that it projects a large Bat emblem on the sky or buildings of Gotham City.

In the stories, the signal is used by the Gotham City Police Department as a method of contacting and summoning Batman to their assistance in the event of a serious crisis and as a weapon of psychological intimidation to the numerous villains of Gotham City. [https://batman.fandom.com/wiki/Batsignal]

For more info: 

"What are you waiting for? The Signal!"
―Commissioner Gordon[ https://batman.fandom.com/wiki/Batman_Returns]