Just kidding. This short chapter tells us all about the Infinite Improbability Drive. But we still can't figure out if it makes any sense.
Basically, scientists always knew how to generate "finite improbability," which was useful for teleporting people's underwear away from their bodies at parties. We wish we had come up with that joke, but it's all Adams.
But they never could create infinite improbability, which would be useful for sending a spaceship across the galaxy in no time at all. The scientists declared this to be impossible, until one student finally realized that an impossible thing was only "a finite improbability"—and since they can make finite improbability, whiz-boom-bang, there's the Infinite Improbability Drive, created out of "thin air."
This doesn't help the student, since the other scientists kill him for being too clever.