This occurs when one vertebra slips forward on another.  It most commonly occurs as a result of the normal ageing process in the spine but can occur as a result of spondylolysis (pars defect) from childhood.  Many people will not require intervention for this but, in some cases, the slip causes pressure on the nerves.  If it cannot be resolved by non-operative treatment, an operation may be appropriate to decompress the nerves and, in some people, this will also require a fusion procedure.