This is just a minor nitpick: Kelvins aren’t the only absolute temperature measurement, only the most commonly used. The Rankine temperature scale is the English unit equivalent - 0 Rankine = absolute zero, and a 1-Rankine temperature change is equal to a 1-degree Fahrenheit change. 0 [sup]o[/sup]R = -459.67 [sup]o[/sup]F.
In order for any of this to make any sense at all, the concept of absolute zero has to be understood. I realize that those posting replies clearly do, so this is my attempt to answer Dave Swaney’s question.
Absolute zero is the temperature at which molecular motion (vibration?) stops completely, like people have said. Temperature, really, is a measure of energy in that motion, so the idea of a temperature below absolute zero simply has no meaning. No more than, say, a stick with negative length. As it turns out, absolute zero is at -459.67 [sup]o[/sup]F, or -273.15 [sup]o[/sup]C. This temperature is defined as 0 K.
What good is this? Well, absolute scales make it easier to compare different temperatures. What’s twice as hot (twice the kinetic energy) as, say, the freezing point of water? 2x0 [sup]o[/sup]F=0 [sup]o[/sup]F? Huh? That should sound suspicious, because it’s not right. Similarly, is -2 [sup]o[/sup]F twice the temperature of -1 [sup]o[/sup]F? No, clearly not - it’s colder.
When your temperature scale is absolute (zero corresponds to zero), you don’t have these problems. 0 [sup]o[/sup]F = 255.4 K, and since Kelvins are absolute we can simply double that value to get twice the temperature, or twice the kinetic energy: 510.7 K = 459.7 [sup]o[/sup]F. Quite a bit different from 0 [sup]o[/sup]F, and also quite a bit more correct.
Hopefully, this is enlightening and not wrong. I’m sure somebody will correct me if I’ve made any errors.