. . . or washboard, as they're known (only?) in the U.S. I've been asked this many times, but this time I'll give a nod to the people at Practical Motoring 4x4 in Australia, who explained it perfectly here.
Of course, knowing how they're formed doesn't make them any less infuriating, especially after 40 or 50 unrelenting kilometers of them has loosened every fastener on the truck and every neural connection in your brain, rendering the slightest first-world problem cause for scarlet-faced rage—such as when the iPhone RAM Mount suction cup pops off the windscreen AGAIN and you catch yourself seriously contemplating heaving the whole assembly out the window.
The only defense short of an airlift out by Chinook is to find that sweet spot speed at which the tires are more or less skipping from one crest to the next. It isn't effective enough to use the word relief, but there is a minor lowering of the rage threshold. Beware, however: With the tires in contact with the earth only about 40 percent of the time, the chances for sudden snap-oversteer on a curve or when braking are compounded. And don't think your suspension is getting the relief you are; it's still being punished.