I was reading a document when an article about atomic clock passed by. There is statement that I don't understand
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
"This definition refers to a cesium atom at rest at a temperature of 0 K"
Could I ask what does it mean by an atom at rest ? Does it mean that the atom is not moving ? If it means "not moving"; how can it be possible?