This is an XML namespace defined in the SMIL Animation Specification. The latest version can be found http://www.w3.org/TR/smil-animation.
For more information about SMIL Animation, please refer to the W3C Synchronized Multimedia 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.