Prime is pretty meaningless across the board to be honest. The only cut that's actually inspected and graded, is the ribeye. And the only part of the ribeye that's actually inspected and graded, is the marbling between the 12th and 13th rib.
I completely ignore it for brisket and ribs.
So I suppose for hamburger, it means that whatever primal was ground up to make it, came from a "prime" carcass. But since hamburger can be made from various cuts, and they don't every specify, then I think it's pretty silly,
Percent fat is probably way more important than the grade of the carcass it came from.