Dental prostheses are worn by millions of people. The loss of tooth starts an immediate destructive process in the upper and lower jaw in which the alveolar bone (the bone whose function it is to retain teeth and support their function) gradually resorbs. Over years, unless the functional loads of teeth can be recreated (for example, by dental implants), this process will continue life long until the jaw comes to resemble an adult-size version of the foetal jaws.
Preprosthetic surgery is the name given to surgical procedures which attempt to make the hard and soft tissues of the jaws more receptive to provision of fixed or removable dental prosthesis.
In the time before successful dental implants became possible, these were designed mainly to make acrylic dentures sit more comfortably and retentively on the rounded alveolar bone covered by firm gingivae (gums) by removing disrupting hard and soft tissue elements, or artificially increasing the area for the denture to sit on (sulcus deepening).
In modern times, the surgery is more about improving that area of the jaws to accept implant-borne prosthesis.