The bearing cover is the plastic or metal centerpiece that protects the steel balls from dirt and grime. It is normally a different color from the rest of the bearing (for an example, ours are red). Some bearings have covers on both sides, in which case it doesn't matter which side they are facing.

Make sure both bearing covers are facing outward from the center of the wheel.

If you do not have new bearings, there is a bearing cleaning guide you can use to make your old ones good as new.