The ground state of the hydrogen atom has the atom's single electron in the lowest possible orbital (that is, the spherically symmetric " 1s " wave function, which, so far, has been demonstrated to have the lowest possible quantum numbers). By giving the atom additional energy (for example, by absorption of a photon of an appropriate energy), the electron moves into an excited state (one with one or more quantum numbers greater than the minimum possible). When the electron find itself between two states, a shift which happens very fast, it's in a superposition of both states. If the photon has too much energy, the electron will cease to be bound to the atom, and the atom will become ionized.