A wheel hub carrier is mounted to the suspension and steering system and as its name suggests it holds or carries the hub assembly. The hub carrier is able to move up and down with the suspension, and also to move left and right allowing the front wheels to steer via movement of the steering gear.
The hub carrier is generally attached to the suspension using ball joints which allow it to move up and down freely as the car travels over the ground. The steering gear is linked to the carrier with a steering arm, also connected using ball joints to allow flexibility as the suspension moves up and down.
A wheel hub carrier is cast piece of solid metal so generally very strong and robust, and as it has no moving parts is generally very reliable. However, a light impact such as hitting a curb can bend or deform the carrier, causing misalignment of the wheel hub assembly.
Additionally the connecting ball joints for the suspension and steering are prone to wear and will likely need to be replaced over time.