What is the ground state in a quantum mechanical model?
Ground state refers to the state when electrons have the most negative energy, that is they are most tightly bound to the nucleus and don't stray very far from the nucleus in course of their orbital motion.
If external energy is gained by the electrons, they may get enough energized to stray far from the nucleus and the energy value becomes less negative. It is then said to be in excited state. Electrons can then move to the lower and more stable states and in that course, they emit a photon of energy equal to the difference in energy of the final and initial states.