Good bearings help you extend the range of your board. The ABEC value is a clue as to how far your board will carry you. A higher ABEC rating indicates a lower tolerance for deviations between the balls of the bearing. More uniform balls allow your board to roll longer distances with the same amount of force applied.
A high ABEC rating does have its downsides however. A ball bearing with a high rating is more vulnerable to grime and dirt, which reduces the lifespan of the bearing and requires more maintenance.