this is a battery pack it powers the circuit
I once burned out this light. for an explanation look at the picture at the top, see the word 3V that means it makes 3 volts so the more battery packs you use the more power you give, it may be hard to see but the light says 2.5V it means that the light is meant for 2 and a half volts. you may have noticed 3V (3 volts) is more than 2.5V (2 and a half volts), its ok because in projects you use more then just a light so it has less power. but if you do need a new piece go to the post need new pieces.