That means that the default namespace's URI is "http://rogers60.com/xmltutorial/2". Every element in the document whose name has no prefix is in that namespace, unless it's specifically declared to be in some other namespace.
when we have another similar line
in the code.
That second line doesn't change the meaning of the first line at all. It means that every element and attribute in the document whose name has the prefix "pub:" is in the namespace whose URI is "http://rogers60.com/xmltutorial/2".
You may find it confusing that elements whose names have the prefix "pub:" and elements whose names have no prefix are both in the same namespace, but that's what it means if both those declarations are in the document. However the default namespace doesn't apply to attributes whose name has no prefix, so the two declarations affect attributes differently.