Wheel bearings that support the hub under load.
These bearings carry weight and cornering force. A worn bearing produces a humming noise that changes in bends. Match the bearing or hub unit by vehicle details and ABS type. Torque the axle nut correctly to avoid bearing preload issues. Reliable wheel bearings are important for safe highway driving.
Noise increases when loading the worn side in a bend.
Most sealed units are greased. Tapered ones may need packing.
Not required but often done to avoid repeat labour.