| Hubcentric is an adjective - it's used to describe wheels that will match up with the center hub of the brake assembly that they're being mounted on. My old 97 GMC 6-lug wheels were like that - the bore in the center of the wheel was a perfect match for the forging on the drum, or the spindles on the fronts. It makes for a more true balance of the wheel relative to the rest of the assembly, and the best way to make sure your wheel bearings last.
Most aftermarket wheels aren't hubcentric, because each manufacturer is going to have a slightly different hub bore, despite having the same bolt patterns on the wheels. Getting a part called a "hubcentric ring" will help to properly center the wheel. Think of it this way:
I tried to fit 97 GMC 6-lug wheels on a 79 GMC 6-lug truck, they wouldn't fit because the newer bores were too small. If I were to try it the other way, there would be some wiggle room with the 79 wheels on the 97 truck because that wheel's bore is much bigger than the hubs or spindles on the truck. It's that wiggle room, no matter how tightly you torque your lugnuts, that can cause play in the wheel to hub assembly, and cause unwanted vibration and problems down the road.
Bnkrupt, in another thread, posted a link to a PDF file that explains it well, and how to go about finding a ring that will fit your truck and wheel combination.
__________________ - Kevin 2003 Sierra 2500HD LB7
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