How/Why does it Glow?

A chemical can be found in its cells. When this chemical is combined with oxygen, it forms a new chemical called oxyluciferin which causes the firefly to produce light.

It glows to find mates and avoid predators

Curiosities:

Firefly light can be yellow, green or orange.

In some places at some times, fireflies synchronize their flashing.

Firefly larvae may glow, even some that live underground or under water. They use the light to communicate to predators that they aren’t tasty (they produce unpalatable, defensive steroids for protection).