Maybe you already know that many living things can make light by a chemical reaction in their bodies. Light made this way is called bioluminescence. We know a lot about the special chemical reaction. But we do not know all about how it makes light.
In the firefly all the bioluminescence occurs at the end of the abdomen, so it might be called a tail light. The firefly can turn that light on and off to make a signal. In the evening the flashing lights you may see from fireflies come mostly from flying males. The females are more likely to be resting. When a female sees a flash from a male close by, she responds with a flash of her own.
Fireflies use their glow as the special means of signaling between males and females.