I think it's someone you can build an offense around knowing they are capable beyond just a journeyman status. In other words you plug in different receivers, different offensive scheme, different coaches and they do well no matter what. It's not just Brady,Manning, Mahomes...it's Hurts, Herbert, Maye, Darnold, Purdy, Goff, maybe Love. Any of those guys in the right circumstances could carry a team to a Lombardi. Mason Rudolph can not....no matter the circumstances.
I mention Mason in the context of the thread which is what we saw from the two young guys in a pre season game. Mason looked All-World at Oklahoma State against bad corner back play in the Big 12...gets to the big leagues...not so much. The guys I mentioned are all "franchise" guys as I define it. And that is; you can build a team around them and know they will perform. You can't build a team around Mason because of his glaring limitations...and there are many Masons in the NFL.