Skateboard bearings mount your wheels to the axle. Maintaining your bearings is essential to getting all that you can out of your board.
All skateboard bearings are the same size. It takes 8 bearings to fully upgrade a board. Two bearings per wheel one on each side of it to keep it stable and to help prevent grinding on the axle. The industry standard size is 608 with an 8 mm bore, a 22 mm outer diameter and a width of 7mm.
Bearings rated under the ABEC scale are typically called "precision bearings," and they are rated with a number from 1 to 9. Higher ratings are given to bearings manufactured against a higher standard of precision. A high number means tighter tolerances, higher quality and a more expensive bearing.