In this particular ad, an astronomer has been waiting all his life to see a bright comet. While he's waiting, he realizes his chewing gum has lost its flavor, and he starts searching through his pockets for another piece. Comets are usually considered to be collections of rock and ice, held together like a loosely packed snow ball, and orbiting the Sun typically in highly elliptical orbits.
Most comets have orbits that take thousands of years to complete one circuit.
Now, when a comet gets near the Earth, it tends to be moving pretty quickly. A typical speed might be 50 or more kilometers per second. For a comet to be really bright, it has to get very close to the Earth. The recent apparition of the comet Hyakutake is a good example. Here was a comet that was easily visible to the naked eye, and it got only 20 or so million kilometers away at closest approach.