Offset Spoke Bed is when you get a rim where the spokes holes aren't in the center... some wide rims, certain disk-specific rims, and so forth have spoke holes that aren't in the dead center of the rim. They can either be both sides further out (for really wide rims, 3" and wider) or disk rims offset both sets of holes to one side. This lets you build a wheel where spokes are closer to the same length on both sides, even if the hub's not centered.