What happens inside an atom that causes it to give off light?

The Photon is the "release" of energy in the form of light. Energy absorbed by the atom causes a change in energy of the electrons of a stable atom. This change forces an electron to move from one valance ring to another. The energy is released in the form of light called a photon. Depending on the amount of energy released and the valance the electron jumps to is what determines the light (photon) wave length. If an atom has too much light than it may disappear/explode.