What's a tooth bone graft?

Bone enhancement: Bone grafting is performed to reverse the bone loss / destruction caused by periodontal disease, trauma, or ill fitting removable dentures. It is also used to augment bone to permit implant placement, such as augmenting bone in the sinus area for implant placement, or augmenting bone to enhance the fit and comfort of removable prostheses, or to enhance esthetics of a missing tooth site.

Bone graft: Adding bone around a tooth.

See below: When a tooth is removed/extracted the socket is like an empty glass. It will partially fill with bone and partially with gum tissue - leading to incomplete healing. A bone graft/bone regeneration will stimulate the bone to more fully heal - filling about 96%+ of the socket/glass.

Unclear?: I assume you meant bone graft around the root of the tooth help regenerate some bone loss? Often periodontal disease can cause bone loss around a tooth and a bone graft is one option.