When any surgery is performed in the mouth, gum heals over the wound very quickly. This is a defense mechanism, as the body tries to re-establish a protective "skin" to prevent outside infection. This healing also occurs after treating a periodontal pocket, and the gum quickly fills any void created by the deterioration of infected bone (See What is Periodontal Disease).
Unfortunately, this does not give the slower healing bone a chance to regenerate, which would restore the pocket back to its original healthy form. With guided tissue regeneration, the gum is excluded from the "wound" by placing a barrier between the gum and the defect, thus keeping the gum out. This allows time for cells in the periodontal ligament and surrounding bone to form new bone.
While complete bony regeneration is rare, there are certain types of pockets that can be predictably restored to a remarkable degree. Your periodontist can tell you if you are a candidate for this procedure.
Defect
Membrane isolating defect so bone has time to heal
New bone forming as membrane dissolves
Final Healing