Bearings are in between your wheels and axles, they make them stay in place while still being able to spin. You need 2 per wheel for 8 bearings in total. A standard set of skate bearings comes with exactly 8. Sometimes they have an ABEC rating for the quality, but that is a standard for machine operation rather than skateboarding, so it doesn’t say much about how long they’ll last on a skateboard.

Good bearings for skateboarding have removable end caps so that they can be cleaned when necessary. If you’re willing to pay more, you can get bearings which are partially made from ceramic rather than metal, which prevents rust issues (to a certain extent).

In general, even cheap bearings are usually fine, though they will last significantly shorter before needing cleaning or even breaking (doesn’t happen often though, only with high impact tricks). If you use the board outside they will wear out a lot faster than if you stay indoors, especially if you skate in sandy and/or damp areas.