This is an unique identifier defined in the Synchronized Multimedia Integration Language (SMIL) 3.0 Specification.
"http://www.w3.org/ns/SMIL" is the SMIL Namespace. The SMIL 3.0 specification can be found http://www.w3.org/TR/SMIL3/
For more information about SMIL, please refer to the W3C SMIL overview. For more information about XML, please refer to The Extensible Markup Language (XML) 1.0 specification. For more information about XML namespaces, please refer to the Namespaces in XML specification.