When your Pi doesn't have network and monitor connected, the cable is the best option to let you connect to your Pi from your PC / Mac.

Features:

• USB to UART TTL
• Red = V_USB, Black = Gnd, White = USB_RX, Green = USB_TX
• V_USB is 5V while TX and RX is 3.3V
• Use newer PL2303TA IC
• Cable length: 1 meter
• Download driver from here