In 1900, Max Planck introduced the quantum concept. When an object radiates light, it releases a unit of radiation energy called a photon.
In 1913 Niels Bohr suggested that electrons travel in circular orbits around the nucleus. The electron possesses a specific energy and it is said to occupy an energy level. If an electron changes orbital in the Bohr planetary model, there is a quantum (fixed jump) energy change. The emission line spectrum results from electrons dropping from higher energy level to lower energy levels. Each time an electron drops, a photon of light is released whose energy corresponds to the difference in energy between the two levels. Einstein and Bohr had a major and highly colourful public disagreement – or perhaps ongoing debate.
In the 1920’s principally because of Werner Heisenberg’s groundbreaking work, our understanding of electrons in atoms became very sophisticated. It was proposed that the energy of electrons can be known only in terms of its probability of being located somewhere within the atom. The description gave rise to the the concept of a quantum mechanical atom. So,a location within the atom where there is a high probability of finding an electron having certain energy is called an orbital. For AS, all you need is The Spectrum of Atomic Hydrogen which you have in your notes.