Bearings are the little ball bearings that sit inside your wheels. They let the wheels spin around the truck axle so you can roll. These small parts matter a lot: they are what keeps your wheels turning smoothly. Look after them and keep them away from dust, sand and water.
The big difference between bearings is the material of the little balls inside. There are bearings with steel balls and bearings with ceramic balls. Ceramic ones are lighter and much more durable, but also pricier. If you’re just starting out, steel bearings are a solid choice depending on your budget. If you’ve been skating a while and want quality that lasts, go for ceramic.
Some bearings also carry an ABEC rating. That was designed for industrial use, to indicate the precision of the bearing: the higher the ABEC value, the more precise and faster the bearing. That mostly holds for industrial applications and doesn’t translate fully to skateboard bearings, because these take a lot of big impacts while you skate. So the same precision isn’t guaranteed as in industry, and a higher rating doesn’t necessarily mean the bearing lasts longer.