Atomic clocks work by exciting atoms as they pass through a magnetic field. This causes electrons to jump up to an excited state.
The excited atoms then pass through a microwave chamber, where they are bombarded with microwave radiation. This causes them to vibrate and emit a specific frequency of radiation.
The frequency of the microwave radiation is carefully tuned until it matches the natural frequency of the rubidium atom’s vibration. This is called the “resonance frequency”.