The orbitals are "quantized" which means they increase in a stepwise manner. For instance, when you climb the stairs, you climb in steps, not between steps.
The ground state is that which is the lowest energy state for an electron—in most cases within the 1s orbital.
An excited state is when an electrons gains enough energy (or is stuck in) an orbital above the 1s orbital, for instance the 2s, 2p, et cetera orbitals.
Impossible electron configurations include those in which the electrons cannot stably occupy their orbitals. For instance, no more than two electrons may exist per orbital per subshell per quantum number.