The circles in the Bohr model represent energy levels where electrons reside around the nucleus of an atom. These orbits are quantized, and electrons can move to higher energy levels by absorbing energy, or drop to lower levels by emitting energy.
The circles in the Bohr model of the atom represent permitted orbits or energy levels in which electrons can reside. According to the model proposed by Niels Bohr in 1913, an electron in a hydrogen atom can exist only in one of these distinct energy states. The closer an electron is to the nucleus, the more tightly bound it is, and it occupies the lowest energy level possible. As energy is absorbed, the electron can transition to higher energy levels, further from the nucleus.
Bohr's Model was a significant step in the understanding of atomic structure because it incorporated quantized orbits for electrons around the nucleus and explained the emission spectrum of hydrogen. Electrons could occupy specific orbitals with certain allowed radii (energy levels) and would emit or absorb photons of discrete energies when transitioning between these orbits.