[Yes, it was a new word to me too.]
Having spoken to a lot of developers about developing for Windows Phone 7, the "Metro" design language is something that is still quite mis-understood.
Today, I read this post about Skeuomorphs which I think is very good explanation of what it means to be "Authentically Digital" and provide "Content, Not Chrome" which are 2 of the principles of metro. Check out the article and see if it helps explain things a bit better?
More about "Metro" can be found in the UI Design and Interaction Guide for Windows Phone 7.