Ground state is the lowest level of energy in a particle, atom or molecule. If energy is added from outside, then the level of energy is increased, and the ground state is changed. Changing ground state is the process of excitement of electrons to higher levels. Any other state is called an "excited state." In ground state, an atom is stable, and does not give out electromagnetic radiation. When an electron drops down from an excited state to the ground state, electromagnetic radiation is released.