The Metadata tag is used to describe the SWF movie in a robot readable form. It will be used by search engines to index your Flash movies.
The f_metadata string is an XML buffer defined using the RDF definition compliant with the XMP specification. You can find more information on the W3C and other websites:
RDF Primer
RDF Specification
Dublin Core
Note that this description can describe everything, from the entire movie to each single line of code in your action scripts.
The string must be UTF-8 encoded.