The Bat-Signal was a searchlight used by the GCPD to call upon the vigilante known as the Batman. The Bat-Signal casts out a huge yellow light which displays the symbol of a bat, due to Batman's insignia being placed over top the searchlight's lenses.
The Bat-Signal, nicknamed Siggy, was first installed by James Gordon when he became the police commissioner. It was designed to let the people of Gotham City know that the Batman was out there protecting them from the deadliest threats. The signal has burned out at least once.