The Bat-Signal is a distress signal device that was used to summon Batman. 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.
The signal was 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. It would eventually go out of operation, with Batman instead speaking with Commissioner Gordon directly over an electronic back channel.
Sometime later, Bruce had miniature Bat-Signals stationed up around Wayne Manor as a beacon to call him. It was only used when Commissioner Gordon needed his help when the Red Triangle Gang attacked Shreck Plaza in the lead up to Christmas. Catwoman was seen looking up at the signal after re-emerging.
Years later, the Bat-Signal had become abandoned, with Batman using a secretive electronic back channel with Commissioner Gordon. Despite not using the signal, the GCPD would still have to replace the glass every week or so due to disgruntled cops smashing it. Shortly after Cecil Culp's call on the Gotham City Council for a vote for no confidence in Gordon at the Gotham City Hall on the evening of November 1st, the night after the riot on Halloween, Harvey Dent and Harvey Bullock flipped the switch for the first time in six months with the batsymbol displaying a crossout as they attempted to summon Batman. The vigilante was being hunted by the National Guard, who had orders to him bring in dead or alive, and the buildings nearby the GCPD headquarters had snipers ready to kill Batman if he approached Dent and Bullock.