An atom has electrons that have certain energy in them and revolve around the nucleus in orbits. Each outer orbit accomodates electrons with energy levels higher than the previous ones. When an electron absorbs energy, let’s say, through heating, then it will jump to a higher orbit and revolve there till the energy is dispensed. This energy is dispensed in the form of a small particle of light called photon. After the photon is emitted, the electron will fall back to its original state.