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 different orbitals. That is, in a ground-state atom, all electrons are in the lowest possible energy levels.
eg: Consider a carbon atom whose electron configuration is the following.
The total energy of the electrons in this carbon atom can not be lowered by transfering one or more electrons to different orbitals. Therefore, this carbon atom is a ground-state atom.
see also excited-state atom