The replacement of missing attached gum is called gingival grafting. The muscle that is pulling down on the edge of the gum is first surgically resected and repositioned away from the gum margin. Then a small piece of attached gingiva is taken from the roof of the mouth, just adjacent to the back teeth, and transplanted to the site in question.
The new tissue reattaches and reforms a new layer of attached gum, which should last a lifetime with proper care. The roof of the mouth heals quickly, just like a skinned knee might. With this procedure the root is not covered, and the tissue stays at the same level as before, except with attached gingiva at the margin. These procedures are very easy on the patient, and rarely require more than over-the-counter pain pills post-operatively (ibuprofen). The most difficult part of the surgery is not chewing on the area for 2 weeks. (See Free Gingival Graft-Clinical Case)
Routine gum grafts ("free gingival grafts") do not cover up the exposed root surface - if this is desired, a different technique is used (See Cosmetic Procedures, Root Coverage). Covering the root does not make the tooth stronger, for the bone, which actually holds the tooth in place, does not change regardless of the new gum level. Rather, root coverage procedures are primarily done for cosmetic reasons, or when there is exceptional root sensitivity after recession.
Recession with a lack of attached gingiva.
Gum graft placed, resulting in adequate attached gingiva and health. The root is not covered.