Hi Patricia. In this case this article can be translated with 'a part', 'a few', 'a little', etc. It's called Articolo partitivo, because it shows just one part or 'some', not because he bought more than one bread, but because this quantity is not specified.
If you would say 'Ha comprato IL pane', or 'Ha comprato I giornali', that would mean that these are more specific.