Read all about what they mean and are for:
http://www.w3schools.com/xml/xml_namespaces.asp[
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Depending on the XML you define it. To your question, defining a URL in XMLNS defines the scema of the xml and gives it a namespace. It does not fetch anything, it's just that one can follow the schema at the given URL.
"The namespace URI is not used by the parser to look up information."