What it Means: “hyperfine levels”
Orbiting electrons create a magnetic field. The direction, magnitude and area of this magnetic field is the magnetic moment (mB).
Some nuclei have mB too. Cs-133 is the only stable isotope of cesium where the nucleus has a mB.
The magnetic moments of paired electrons cancel each other out. Cesium has an unpaired electron whose mB can interact with the nucleus.
Hyperfine refers to the interaction of a nucleus and electron mB . There are two hyperfine energy levels because they can interact in two ways. The magnetic fields may repel each other or attract. Repelling is higher in energy for the atom, attracting is lower.