ABEC is an industrial tolerance standard. It describes manufacturing tolerances for certain bearing components, not a complete score for skateboard speed, durability or suitability.
Why the number is not the whole story
A skateboard bearing also depends on materials, seals, lubrication, alignment, wheel setup and the surface you ride. A higher ABEC number does not automatically make a skateboard roll faster.
What should you buy?
Choose a bearing that is well made, skate-specific and easy to maintain. ABEC 5 is a sensible everyday choice for many skaters; ABEC 7 may suit buyers who specifically want that rating, but it is not a shortcut to better skating.