You might know it as "possie traction" or "spools" in the hot-rod world. In most vehicles, only one tire is actually driving the vehicle at a time (the one with the least traction). This is GREAT for on-road driving, as it keeps the tires alive longer, and there's enough traction between the rubber & the road to only need one tire getting traction.
In 4x4 world, if you're going through mud / snow or other low-traction areas, the tire with the least traction spins - sometimes freely. Or, you'll be driving over an obsticle (rock, log etc), and lift one of the tires enough, that it'll tend to free spin. Unfortunately in these situations, the tire with the most traction just sits there. The way around this is lockers. There's a few locker choices, I'll group them by how they work (and least priced to most expensive):
* Welded (aka lincoln), or spools
* Auto lockers (lockrite, or EZ-locker)
* Selectable (KAM, or ARB)
There's a LOT to discuss between these types of lockers. Lockers are discussed every week or two, and rather than re-hashing what's already been discussed, and diluting the info that's already out there... I'd prefer you do a search, find a thread that's already discussing these locker types and post any questions you may have in those threads.