A custom power cable allows to power a BeagleBone via its barrel jack connection, e.g. from a USB powerbank. This allows to use USB devices with power ratings exceeding the 2 Amperes at which the USB power regulator maxes out. To do this, simply cut of the non-Type-A plug of a USB cable and solder a barrel jack to it. Typically, the wires within the USB cable are color-coded with red being positive and white black being ground.
Always test the cables with a voltage meter, e.g. by plugging it into the USB port of your computer and measure the wires against each other!
The barrel connection on the BeagleBone Black is center-positive.