These control the spin of your wheel and can greatly improve your push efficiency
Most bearings are measured by an ABEC rating. The higher the ABEC rating, the more accurate and precise the bearing will be. This rating system includes grades 1, 3, 5, 7, and 9. The ABEC rating does not specify many critical factors, such as load handling capabilities, ball precision, materials, material Rockwell hardness, degree of ball and raceway polishing, noise, vibration, and lubricant. Due to these factors, an ABEC 3 classified bearing could perform better than an ABEC 7 bearing.
The wheels on a board are specific to the surface you want to skate, for instance a softer wheel can be used outside for better rollover and a harder one can be used to slide and maneuver on smoother surfaces better.