The ABEC scale is essentially a measurement of bearing tolerance for high precision bearings as dictated by the Annular Bearing Engineering Committee (ABEC). The scale runs from ABEC 1 through ABEC 9 using odd integers only. Essentially, the higher up on the scale a bearing is rated the tighter the tolerance is.

Tighter bearing tolerance means that the bearing moves with less resistance at high speeds, giving you a bit more roll for your push. In general most ABEC 3 or ABEC 5 bearings have steel races (shell of the bearing) with hardened steel ball bearings, and most ABEC 7 or ABEC 9 bearings feature steel races with super hard (much harder than steel) ceramic or titanium bearings.