Metropolitan France
The expression refers to continental (or, perhaps better, European) France, as opposed to overseas areas of France.
The non-metropolitan parts of France are all former colonies. Five are full-fledged departments. For heaps more information on the subject, consult this Wikipedia article and associated articles.
The word means “mother-city.” In ancient Greek it referred to the relationship between a colonizing city-state such as Athens and its dependent colonies. (See this other Wikipedia article.)
This use of métropolitain(e) is fairly modern. English-speaking colonizers may have used it before French-speaking ones adopted it.
