Can you tell me something about the simplest type of dental bone grafting available if bone level is low on a tooth? Or, is a big operation needed?

Simple Surgery. A periodontist, or a general dentist that does periodontal surgery, needs to evaluate if you are a candidate and what type of surgery to use to clean away the diseased areas and placement of a bone graft material to regerate the missing bone sufficient to attain a healthy condition.
Flap procedure. The periodontist will make an incision, push the gum tissue back exposing the pocket in the bone, clean out any diseased tissue, make sure the tooth is free of deposits, place the bone graft material, sometimes cover it w a membrane, and then suture back the gum tissue. It is a skillful procedure done gently and efficiently w little post op pain.
Bone Augmentation. Bone grafting has taken great strides in the last ten years. Simplest is to graft the site when the tooth is extracted. I prefer a donor bone graft today but if the patient is amiable to an extrusion of the tooth to be lost, then i'd extrude the tooth with ortho- brackets. That would grow bone that way too(not the simplest). So grafting ; implants are complex care. Get a couple of opinions. Best.