In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called as orbits or shells. Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.
Niels Bohr explained the line spectrum of the hydrogen atom by assuming that the electron moved in circular orbits and that orbits with only certain radii were allowed. The orbit closest to the nucleus represented the ground state of the atom and was most stable; orbits farther away were higher-energy excited states.