The Total Perspective Vortex is a device built as a practical application of the theory of atomic interactivity, the idea is that if every atom of the universe is affected by every other atom of the universe, it is theoretically possible to extrapolate a model of the entire universe using a single piece of matter as a starting point. The TPV does this using a piece of fairy cake as its extrapolatory base.
Originally created by its inventor Trin Tragula as a way to get back at his wife (who always told him to get a "sense of proportion"), the TPV is now used as a torture device on the planet Frogstar. In a small chamber, you are presented with a 3-D model of the entire universe in exacting detail. In that model is a microscopic dot with the legend "you are here" printed on it.
The only person to survive exposure to the TPV is former President of the Galaxy Zaphod Beeblebrox. At the time he was exposed to the TPV, he was inside a computer-generated universe created for his protection by Zarniwoop. Since the entire universe was created for him, the TPV effectively told him that he was, in fact, the single most important person in the universe. This allowed Zaphod to survive the experience, and also did not surprise him in the least.
He then ate the fairy cake. This theoretically rendered the TPV inactive, until such time as a new piece of fairy cake could be delivered.