C# .NET - dumb question of the day what does XML facade??

I am reading msdn articles on xml and I am coming across the word xml facade
frequently.

here is the context:

The fact that an XmlReader doesn't require that the entire XML document be loaded in memory and is read-only, makes it a good choice for creating an XML façade over non-XML data sources. 

what does xml facade means?

thanks
nath