The Sources Screen lists NewsBrain's article sources. You can see it by selecting the Article Sources item on the Settings Screen.
NewsBrain uses Feedly as it's main source of articles, so the permanent, built-in Feedly item is always there. This is all you need.
In special cases you might want to add additional sources, but it is unusual and unnecessary, as Feedly lists millions of sources from all around the Internet. You might add a private RSS feed page, an regular HTML web page, or even a text file URL. Again, this would be highly unusual.