ABEC, Annular Bearing Engineering Counsel, is a rating system used by the bearing manufacturers (ABEC 1, 3, 5, and 7). Usually, the higher the rating the more precise and smooth the ride is.
The ABEC rating does not, however, determine the speed of the wheel and is in no way a measure of the quality of the wheel. The tag on the shoes will tell you what ABEC rating they are.