Apparently this means "above everything else" and is derived from the German*, but one can only get at that definition via Merriam Webster by looking it up as "uber alles." Yet the definition there has the first character of as ü rather than u. Go figure.
Also, apparently if you have this capitalized, as Über Alles, it means something totally different (see the Wikipedia entry here).
* German language words (sorry) will always make me a bit nervous.