Bohr's atomic theory suggests that atoms have a positively charged nucleus that is orbited by electrons that move in circles. These electrons move in specific, distinct orbital levels. This theory has since been made obsolete but has had a lasting impact on the understanding of atomic structure and quantum physics.
Quantum theory claims that energy can only be transferred in set quantities. Bohr utilized this concept while coming up with his hypothesis. Bohr suggested that electrons can move and be transferred, but they can only do so in certain specific orbits. This model of the atom and its subatomic particles is often compared to the solar system with the electrostatic forces of the atom acting as the attractive force rather than gravity. Bohr's model was a modification of the prevalent theory at the time, the Rutherford model of the atom, which suggested a dense, positive nucleus but did not include Bohr's multiple specific orbits.
Bohr's model has been refined and improved since it was originally conceived. Eventually, physicist Arnold Sommerfeld created the Bohr-Sommerfeld model of subatomic particles, which suggests that the orbits of electrons are elliptical and not circular. However, this model also had several inconsistencies. Bohr's atomic theory and his model of subatomic particles has been largely replaced by modern quantum mechanic theory in the academic world.