The Bat-Signal is a distress signal device to draw 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 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.
Batman gives the signal to the police after the death of The Joker as a gift so that when he is needed they can summon him. The film ended with Batman looking at the signal while standing on a gargoyle.
Bruce has miniature Bat-Signals stationed up around Wayne Manor as a beacon to call him. It is only used when Commissioner Gordon needs his help when the Red Triangle Circus Gang attack Shreck Plaza in the lead up to Christmas. Catwoman is seen looking up at the signal at the end of the film.
The criminal psychologist Dr. Chase Meridian uses the Bat-Signal to call Batman, in order to seduce him. Batman is slightly peeved at her usage of the Bat-Signal as a mere "beeper". Later, the Riddler alters the Bat-Signal by projecting a question mark into the sky with the first symbol forming the dot at the base. The signal was restored when Batman flew the Batwing right though it much to Commissioner Gordon's delight. The film ended with Batman and Robin running in front of it.
Poison Ivy alters the Bat-Signal by changing it to a "Robin-Signal" to lure Robin into a trap. However, Robin was able to trick Ivy into revealing Mr. Freeze's plan to freeze Gotham City. In the film's ending the signal was revealed to be restored when Batman, Robin and Batgirl were running in front of it.