The word "prime" is a quality grade given by the U.S. Department of Agriculture to describe the highest quality beef and other meats, such as veal and lamb, in terms of tenderness, juiciness and flavor.
Less than two percent of all beef produced in the United States will earn the prime designation. You will probably never see prime meat for sale at the grocery store; rather, it tends to be purchased by high-end restaurants, hotels and, of course, Schatzies. 
This meat quality grade is given based on a combination of marbling and maturity. Marbling, or flecks of fat within the meat, adds flavor, and younger beef produces the most tender meat. 
Therefore, the "prime" grade will be given to meat that comes from the youngest beef with the most abundant marbling.
Like I always say, I'm a butcher, I'm not a chef and I don't speak French. But I do know a beautiful piece of meat when I see it.