Wheel bearings are what allows each wheel to rotate. Depending upon the type of wheel bearing they either require no maintenance or can be disassembled, cleaned, and inspected for wear. A wheel bearing replacement is needed if the wheel bearings show signs of wear due to them becoming loose and and develop excess play. Also, a failing bearing may be put under load and make a distinctive growling noise when the wheels are in motion. That is usually a sign that the bearings need replacing.
Some vehicles have serviceable wheel bearings. In order to service the wheel bearings, the wheel must be removed. Then the brake drum, brake rotor, or wheel hub assembly is removed. Then the wheel bearings are removed, cleaned, and inspected. If they show no signs of wear, then they are packed with fresh grease and re-installed. If they show signs of wear, they should be replaced.
A wheel bearing replacement is covered under most vehicle's factory warranty. However, when the factory warranty expires there is no coverage for the wheel bearings. An extended auto warranty is a great way to maintain coverage for not only the wheel bearings, but also other parts of your vehicle.