It is a fracture of part of the structure of your spine, usually the lower part.
These type of fractures can be genetically caused. However, in my world of legal/medical causation, they are produced from job injuries. Specifically, strenuous lifting and twisting of heavy weights. Most orthopedic surgeons will try to say genetics caused this fracture. Do not believe them! We look for causes that stem from activities at work. Repetitive trauma or multiple lifting attempts over a prolonged period of time can be the source of a pars fracture. This, too, could readily be due to one's job duties over time.