It likely means the rim is out of the lateral plane in some spots by up to the amount specified. The wheel appears to wobble when you spin it. Rim flatness or trueness is very important, but is easily adjusted by tightening/loosening opposing spokes to pull the out-of-true sections into plane. A well built and maintained wheel will have no lateral run out greater than a small fraction of a mm.

Alternatively, but not likely, the seller could be referring to radial true, which refers to the roundness of the wheel. When a wheel is radially out of true, it appears to "hop" when you spin it. Similarly this can be adjusted by spoke tightening and loosening.

These types of maladjustments are not reasons not to buy a used wheel, so long as you have an understanding with the seller that the sale is conditional upon a qualified mechanic being able to restore near perfect true to the wheel. This adjustment should only be done by someone who knows how so that in pursuit of nearly perfect true, he doesn't totally screw up the tension balance among the spokes. One doesn't just start twisting spokes nipples without understanding how it all works.

Finally from time to time (more frequently with aluminum rims than carbon) the true cannot be properly restored because the rim is bent or dented or not flat to begin with. That is not a wheel you would want to buy.