The source domain indicates the name of the website on which information was posted. From January 8th the list of sources you have used is visible directly inside the My Reader interface. And, what's more, you are able to use this list to filter information. This allows you to create reports or run analysis based on the information from one source or a selection sources.
The number of pieces of information from each source is also given, so you can quickly see where your information is coming from.