What thing gives off light?

Some natural sources of light are lightning, phosphorescent fish and worms, and some fungi.

Stars give off light, due to atomic processes. When an atom is heated, some of the electrons will move to a higher energy state, and when these descent to their 'natural' orbit, they give off light quite characteristic of that atom and that electron transition.

By studying this, we may determine the temperature and composition of quite distant stars.