Lanap dental procedure was minimally successsful at 1 tooth due to a furcation diagnosis. Please fully explain in layman's terms. Also, how common?

Avascular tissue. The area around the roots on a molar tooth (furcation) has very little blood supply, and for this reason, is a challenging area in which to achieve regeneration. Depending on what else may have been done to the tooth (root canal etc) it may have a fracture perpetuating the problem. Sometimes removal and replacement with a dental implant is advised in these frustrating situations.
Laser TX. There are a range of opinions on the efficacy of laser tx in periodontal disease. The disinfecting result is very well backed, the reattachment of tissue or lessening of pockets is not so universally accepted. Over all with a furcation involvement or any compromised periodontal area, ease of cleanliness, amount of existing present bone and patient health and compliance is the key to success.
Furcation is. .. Furcation is the crotch of the tooth between two roots that should be covered by bone and gum tissue but is not in various stages of periodontal disease. The more exposed the furcation is and the more difficult it is to clean it out thoroughly, the more prone it is to getting worse. The best success with laser treatment or any other is with a so called "three walled bony pocket".
Crotch. A furcation is the area of a multi-rooted tooth where the root anatomy blends into the crown. When buried in healthy bone, this is just descriptive anatomy but when recession or periodontal disease exposes this 'crotch' area, it becomes a very difficult area to properly clean and manage, greatly complicating periodontal health and thus a favorable outcome of your lanap.
Usually hopeless. You really should discuss this with your own dentist. However, when bone is lost between the root of a multi-rooted tooth, one does not have enough surrounding bone to help fill the defect, regardless of the type of surgery performed. This situation is usually hopeless and can only be maintained, a root resected off, or extract the tooth. There are no procedures with consistent good results.