This short video, made at the UK's National Physical Laboratory, proudly explains the science behind the world's first accurate atomic clock. It was designed by Louis Essen and built at the National Physical Laboratory in 1955, and the video is worth watching for the voiceover alone.
As the narrator explains: "The essential parts are not too complicated, and the principle is easily explained." He's right, of course, but his delivery makes it sound like what follows will require a PhD in physics to understand. Don't worry, it doesn't; in fact, the concepts behind the clocks that now crop up everywhere from fancy science labs to inside expensive preamps are really quite straightforward.