It means that there are rotating shifts around the clock in eight hour segments. For example, in hospitals, the usual shifts are 7 a.m. to 3 p.m., 3 p.m. to 11 p.m., and 11 p.m. to 7 a.m.
Some stores, factories, or restaurants may only have two eight hour shifts daily and be closed the other 8 hours, or they may be open 24 hours daily and have three eight-hour shifts.