“xmlns” means “XML namespace”. In such a namespace document you can define the elements you want to use in your document. For example, XHTML is a namespace of an XML document (you might have noticed the
xmlns="http://www.w3.org/1999/xhtml" on the
<html> element in XHTML documents). If you have similarly named elements in your XML document you can differentiate them by assigning a namespace.
Wikipedia has a good article about that.