Joints damaged by arthritis become uneven, thin, and worn down. Replacing the cartilage and the subchondral layer of bone just under the cartilage is an example of resurfacing.
'Biologic' resurfacing means that live tissue grafts are used. The layer of bone and cartilage used is called an osteochondral allograft. Allograft means it comes from a donor after death.
Synthetic resurfacing would replace the damaged cartilage and bone with a thin layer of plastic such as polyethylene used with a joint replacement.