The quick and easy explanation: Choose the Channels on the left you wish to render by double clicking them. This will auto-create a new display driver with the same name to be rendered.

If you want a smaller list or maybe single file output with these channels, be sure and have a Display Driver selected on the right when you add the Channels using the arrow-to-list button on the bottom left of the Channels box.

You must use a file format in the Display Driver that will hold these channels, we recommend leaving the default of OpenEXR nearly 100% of the time. If you do not know what file types you might need, leave it at default.