A Certificate of Occupancy (CO) is issued by the local building department once a project passes its final inspections, certifying that the building complies with code and is safe to occupy for its intended use. Without it, an owner generally can't legally move in or open for business.
Getting to the CO drives the tail end of the schedule: the fire, building, and health inspections all have to pass, which means the punch and the life-safety systems have to be done. A temporary CO (TCO) sometimes allows partial occupancy while a short list of items is finished.