You and your doctor have decided that the pain in your knee has gotten so severe that non-surgical management has not been helping. By replacing the joint surfaces of the knee, the arthritis and its associated pain will be taken away, and you will be able to move better.
The knee is a joint made up of three bones: the femur (thigh bone), tibia (leg bone) and patella (kneecap). They both have joint surfaces that meet each other, and will be replaced. The actual replacement involves cutting off the diseased bone and putting metal in its place with plastic.