Extra bare wire in USB cable
This is a shielding ground wire (or S-GND) which is left bare on purpose, so it makes contact with the foil. It has to be connected (crimped or soldered) to the metal casing of the USB receptacle / plug at the end of the cable. If your device includes a USB connector, S-GND can be connected to the device ground (same as GND), but this connection is optional.



The reason this wire is used in addition to the foil is that you cannot crimp or solder alunimium foil reliably. On the other hand, making the whole shielding out of copper (i.e. copper foil or woven copper shield) would be too expensive.