ABEC is the Annular Bearing Engineering Committee.
Bearings are rated using an odd number between 1 and 9. The higher the number, the greater the manufactured precision of the bearing. However, the ABEC rating does not specify many other critical factors, such as smoothness of the rolling contact surfaces, ball precision, and material quality and most important to skateboarding is side loading and impact resistance.
The higher the number on the ABEC scale means that th bearing will perform to its optimum speed which is much higher than would ever be encountered in skating.
For skate bearings, the ABEC rating is for the most part meaningless. The higher number means a higher quality bearing, but it doesn't ensure the best skate bearing.
All Bones bearings, most Swiss and ceramic bearings do not have an ABEC rating because they are designed specifically for skateboarding.