What does it mean to have an element in a ground state

A ground-state atom is an atom in which the total energy of the electrons can not be lowered by transferring one or more electrons to any more lower shells.

For eg If an atom's electron is at n=2 energy level for ground state it can no longer send that electron to any lower energy levels. It can be sent to higher energy levels which means it will no longer be in ground state.

Basically, if you think about it this way where "ground is lower than everything you cannot go below ground then you might be able to remember what ground state means."

Explanation: