It's a section of meat low on the steer between the brisket in front and the flank behind. Meats from that area tend to have a very good beef flavor and are suitable for grinding.
The selection of chuck, hanger and short rib meat should be a flavorful burger. However, since we don't know which cuts are in the ground beef we buy OTC, a difference in flavor may or may not be noticeable.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan