The 0.5mm Pitch 51 Pin Ribbon Flexible Flat Cable is a very thin flat cable that is commonly seen in high-density density electronic applications such as laptops and cell phones. To simplify insertion or to give strain relief, each end cable is strengthened with a stiffener.
The pitch of an FFC is the distance between the centres of one conductor and the centres of the adjoining conductors, which in this cable is 0.5mm.
The stiffener increases the thickness of the cable somewhat. This cable is used to connect a peripheral to the Raspberry Pi ribbon connectors, such as an external display, camera, or the AlienSpec Ribbon Connector Breakout Board.