What's a wheel bearing?
Wheel bearings basically let your axle spin freely, by reducing drag. They are usually made up of rollers (or bearings) that are pressed in between two rings.
Why would I want to replace them?
Over the years, your wheel bearings can wear down. The truth is that you probably won't need to replace them, but instead repack them. Repacking a bearing just means to gut more grease in it.
What you need