This hard drive cable carries the data and power to the 2.5" SATA hard drive. Two in one! Well, I guess technically it's two in two; the power and data cables are separate but wrapped together, and generally referred to as a single cable.
923-00035 A1418 Hard Drive Cable for iMac NB-5877W This is a replacement hard drive cable for the Late 2013 and Mid 2014 iMac 21.5". Apple part number: 923-00035