Electrons can be found in the ground state. When energy is added to the atom electron can jump to higher energy levels. The electrons are considered in the excited state. These electron can relax down to a lower energy level and release the energy as light.
Early on, scientist believed that atoms can exist on any energy levels. Because only certain photons are emitted. We know that only certain energy changes are occurring. There are certain discrete energy level that will always emit photons. There are quantized energy levels versus continuous energy levels