Atoms have characteristic oscillation frequencies.
Perhaps the most familiar frequency is the orange glow from the sodium in table salt if it is sprinkled on a flame. An atom will have many frequencies, some at radio wave length, some in the visible spectrum, and some in between the two. Cesium 133 is the element most commonly chosen for atomic clocks. To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately. This is normally done by locking a crystal oscillator to the principal microwave resonance of the cesium atom.